Inr.J. Biochem. Vol. 23, No. 11, pp. 1175-1183, Printed in Great Britain. All rights reserved

1991

0020-71IX/91 $3.00+ 0.00 Copyright 0 1991Pergamon Press plc

MINIREVIEW ENERGY

METABOLISM

AND THE SKIN

D. T. NGUYEN and D. KEAST* Department of Microbiology, University of Western Australia, QE II Medical Centre, Shenton Park, Nedlands, Western Australia, WA 6009, Australia (Received 30

November

1990)

INTRODIJCHON

STRUCTURE OF

The skin has been shown to be one of the largest

mammalian organ systems. Occupying around 10% of the body weight, it is complex in both structure and function. In addition, the skin has been shown to be directly involved in the control of water retention, thermoregulation and production of essential vitamins, providing a protective barrier to internal organs against constant physical abuses (Odland, 1983). It was established early on that anaerobic metabolism appeared to be the main source of energy in the skin with aerobic metabolism playing a minor role (Frienkel, 1960; Johnson and Fusaro, 1972). High activity of the pentose phosphate pathway was also observed in growing and regenerating skin and it was suggested that the main role of this pathway was the synthesis of the carbon backbone essential for nucleic acid precursors which are required by the skin (Halprin and Okhawara, 1966a; Im and Hoopes, 1970; Foreman et al., 1979). The skin has also been shown to utilize fatty acids and glycogen as energy fuels only when its primary substrate, glucose, is severely restricted or when extra energy is required (Cruikshank ef al., 1962; Im and Hoopes, 1970; Cooper et al., 1976). Recently, significant oxidation, as measured by the activity of glutaminase, has been detected in the skin of both mice and rats throughout their lifespans and under various dietary conditions, as well as through the healing of full thickness wounds (Keast et al., 1989; Nguyen and Keast, 1990a,b). Glutamine is known to be utilized both as an energy source and/or a nitrogen donor source for purine/pyrimidine formation in a wide range of tissues (Reitzer et al., 1979; Leighton et al., 1987; Griffiths and Keast, 1990), where it is often an absolute requirement (Ardawi and Newsholme, 1985; Ardawi, 1987; Griffiths and Keast, 1990). If glutaminolysis functions in the skin in the same way as in other tissues, it would suggest that energy metabolism in the skin may be a much more dynamic process than previously thought, and may be of critical importance under deficient dietary conditions.

*To whom all correspondence

should be addressed.

THE

SKIN

The structure of skin has been well documented (Odland, 1983). This short review represents an attempt to simplify its description and the main functions of different cell populations within the skin. The skin has been divided into three layers: the epidermis, the dermis and separating these two is the basal cell layer (Fig. 1). The epidermis

The epidermis has been shown to be a thin 75-150pm stratified epithelium, 95% of which is keratinocytes (Odland, 1983). As the keratinocytes migrate upward from the basal cell layer, they form different layers of the epidermis. The stratum spinosum, the stratum granulosum and the stratum corneum (Fig. 1). The epidermis has been thought to obtain its nutrients and oxygen and discharge its waste products by diffusion across the epidermal-dermal interface into a network of dermal capillary loops (Braverman and Ken-Yen, 1977, 1981). Under normal physiological conditions, the epidermis has been shown to be in a constant state of turn-over or self-replacement which takes around 26 to 42 days (Halprin, 1972). Although, the epidermis is only 5% of the skin mass, it has been shown to be a highly metabolically active tissue compared to the dermis (Leibsohn et al., 1958). It has been recognized that not only are there 3-4 fold differences in the respiratory activity between the epidermis and the dermis of the skin but the level of respiratory activity is also different within different layers of the skin epidermis (Leibsohn et al., 1958). Aside from serving as building blocks for the epidermis, keratinocytes, under stimulation, have been shown to produce and secrete many soluble factors which can influence many biological functions not only of skin cells but also cells in circulation. Some of these cytokines include interleukins (IL 1, IL 3, IL6, IL8) (Sauder et al., 1984; Gallo ef al., 1989; Kirnbauer et al., 1989; Larsen et al., 1989), colony stimulating factors (Macrophage-CSF, Granulocyte_Macrophage-CSF) (Kupper et al., 1986; Chodakewitz et al., 1989), transforming growth factors (TGF alpha and TGF,) (Ansel et al., 1990) and tumour necrosis factors (TNF) (Kock et al., 1989).

1175

D. T. NGUYENand D. KEAST

1176

stratum corneum stratum granulosum

E EPIDERMIS

stratum spinosum

stratum basale

i BASALCELL

C

LAYER

capillary loop DERMIS

Fig. 1. An outline

of the structure

The Langerhans cell is the second most important cell type in the epidermis. Epidermal Langerhans cells which have the characteristic Birbeck granule, have been shown to be the most important antigen presenting and accessory cells in the skin (Streilein, 1983; Bos and Kapsenberg, 1986; Steiner et al., 1985). It is becoming accepted that this population of cells can pick up and carry antigens and present them to T-lymphocytes in the regional lymph nodes (Schuler and Steiman, 1985; Teunissen et al., 1990). Once outside the epidermal environment, they have been shown to undergo significant changes to resemble veiled or interdigitating cells (Schuler and Steiman, 1985; Teunissen et al., 1990). Epidermal Langerhans cells have also been shown to produce and secrete IL 1 (Sauder et al., 1984). Other migrant cell types, such as the melanocyte and the Merkel cell are also present in the epidermal layer of the skin (Odland, 1983). While the main role of the melanocyte is the production of melanin which controls the degree of epidermal photoprotection and pigmentation (Gordon et al., 1989), Merkel cells have been suggested to serve as targets for the ingrowth of nerve endings during embryogenesis (Pasche et al., 1990). It has been shown that the growth, dendricity and melanization of melanocytes was regulated by soluble factors produced by keratinocytes (Gordon et al., 1989). Although non-malignant melanocytic cells have been shown to produce quantitatively and qualitatively reduced numbers of soluble factors, melanocytic cells have been shown to produce and secrete both IL1 subclasses, IL6, IL8 and GM-CSF (Bennicelli et al., 1989; Gyorfi et al., 1990). Murine epidermis has been found to have a novel population of Thy-l+ epidermal cells (Tschachler et al., 1983; Bergstresser et al., 1985). Although this cell population has not been detected either in human or rat epidermis, it is suggested that it is closely related to the “double negative” (CD4-/CD8-) gamma/delta TCR T-lymphocytes which has been shown to present in the intraepithelial area of the skin and intes-

of the skin.

tine (Borst et al., 1987; Bonneville et al., 1988; Goodman and Lefrangois, 1988). It has been suggested that Thy-l + epidermal cells could act in immune surveillance and function as the primary line of defence in the skin by recognizing highly conserved proteins such as heat shock proteins (HSPs) which are released by infected, damaged or transformed skin cells without the requirement of recognizing every specific antigen imposed on the skin (Arsanow et al., 1988). The dermis

The dermis makes up the main mass of the skin. The structure of the dermis is a dense fibroelastic connective tissue which has been shown to be made up of collagen fibres, elastic fibres, glycosaminoglycans, salts and water (Odland, 1983). The dermis has also been shown to support extensive vascularity, nerve networks and specialized sweat glands and hair appendages (Odland, 1983). The main structural cell type in the dermis is the fibroblast, however, other migratory cells such as mast cells and tissue macrophages are also present (Odland, 1983). The dermis has been shown to be essential for the maintenance of the epidermis. Degeneration of the isolated epidermis in vitro has been observed in the absence of the dermis (Briggaman and Wheeler, 1968). In addition, cultured, epidermal cells which if grown on “substrates” other than the dermis have been shown to lack the Malpighian and granular cell layers which are essential for the formation of the stratum corneum layer in the epidermis (Briggaman and Wheeler, 1968). ENERGY

REQUIREMENT

IN THE SKIN

Like any other organ, the skin has been shown to require nutrients for the production of energy to sustain its structural and functional integrity. Glucose, up to now, has been thought to be the main fuel of the skin where it has been shown to be used mainly

Energy metabolism and the skin through anaerobic glycolysis (Johnson and Fusaro, 1972). However, when the host is subjected to protein and/or energy deficiency for a period of time, an adaptation period occurs in which there is a redistribution of fuel sources among all tissues in a way that favours the working muscles (Cherel et al., 1988). Under these circumstances, as glucose becomes depleted and reserved solely as the fuel source for muscles (Cherel et al., 1988) it is possible that the skin has to switch to other fuel sources. Surprisingly, very little is known of the biochemical make up and the dynamic adaptation of the use of different fuel sources of the skin, under conditions such as dietary stress or during wound healing. Glutamine, which has been shown to be present in high concentrations (20%) in the plasma amino-N pooi, has been shown to serve as both an important fuel source and as the main supplier of nitrogen for the formation of purine and pyrimidine bases in many cells (Haussinger and Sies, 1984; Meister, 1984; Bulus et al., 1989). Glutamine has been shown to be used concurrently with glucose as a fuel source in rapidly dividing cells such as fibroblasts and Hela cells (Reitzer et al., 1979; Wolfrom et al., 1989) in tumour cells (Kallinowski et al., 1987) or in cells with a high protein secretory activity such as enterocytes, pulmonary endothelial cells (Leighton et al., 1987) macrophages (Newsholme et al., 1986) and in cells of the lymphoid system (Ardawi and Newsholme, 1985; Ardawi, 1987; Griffiths and Keast, 1990). Even though the role of glutamine as a fuel source and as a nitrogen supplier has been explored in these cells, its role in the skin in viro has only recently begun to be investigated (Keast et al., 1989). It has been suggested that, from the results of this preliminary work, glutamine could also play a very important role in dividing epidermal cells during the healing process in normal skin and especially in skin of animals subjected to dietary stress. GLUCOSE

METABOLISM

Glucose which is used as an energy source for all cells in the body, has been shown to be supplied to the skin directly from the diet and/or gluconeogenesis in the liver (Exton and Park, 1967; Mallette et al., 1969; Aikawa et al., 1973). Furthermore, glucose has been shown to be the essential fuel source for erythrocytes and the central nervous system (Rapoport, 1968). Glucose utilization has been thought to occur firstly through anaerobic glycolytic respiration, to form pyruvate and then through aerobic respiration via the Krebs cycle for maximum generation of ATP. However, it has been shown that the utilization of glucose through anaerobic glycolysis is the main energy pathway in the skin, although all enzymes of the oxidative TCA cycle are present and fully functional (Frienkel, 1960; Johnson and Fusaro, 1972). Nearly 70% of the assimilated glucose in human epidermal slices has been found to convert to lactate via the glycolytic pathway compared to only 2% of glucose being used through the TCA cycle (Frienkel, 1960). In addition, more glucose was found to be used in the skin through the pentose phosphate pathway than via the TCA cycle in the same experiment (Frienkel, 1960).

1177

Increases in glucose utilization has been observed in growing hair follicles (Adachi and Uno, 1968), in wound healing of skin (Im and Hoopes, 1970) and in the psoriatic skin (Halprin and Ohkawara, 1966b). It has been well documented that gluconeogenesis is the most important function of the liver, generating glucose from lactate, pyruvate, glycerol or amino acids (Exton and Park, 1967; Mallette et al., 1969; Aikawa et al., 1973). This has been observed during fasting, starvation or heavy muscular exercise (Holt et al., 1961; Morgan and Parmeggani, 1965; Exton and Park, 1967). There is little information on the fate of lactate produced by the skin. One of the main reasons has been the gross underestimation of the rate of lactate/glucose interconversion. It has been suggested that while some of the lactate produced by the skin, could serve as an anti-bacterial agent, since it was excreted as a component of sweat, the rest could take part in the Cori cycle (Johnson and Fusaro, 1972). Briefly, lactate produced by the skin, enters the blood stream, and is taken up by the liver for gluconeogenesis. This would suggest an active role for the skin in the control of the plasma blood glucose level. Wolfe and co-workers (1978) have indicated that although there is an increase in lactate production along with high glucose turn-over rate in the burned-skin of the guinea pig, the arterial glucose level still remains unchanged which suggests the presence of a Cori cycle flux to the liver. However, even though there is no evidence, as yet, of the direct involvement of lactate produced by the skin in gluconeogenesis in the liver and attempts have also failed to show that such activity exists in wounded skin (Daley et al., 1988) the importance of lactate, produced by the skin, should not be underestimated. It has been shown that lactate concentration in the skin can exceed that of the blood and that the total pool of lactate in the body is about 2 g. In addition, the release of lactate accumulated in the skin has been suggested to be one of the main contributing factors in the metabolic acidosis which can induce shock in the host (cited in Johnson and Fusaro, 1972). Glucose metabolism during prolonged starvation When an individual is subjected to protein and/or energy deficient conditions for a long period of time, a redistribution of the use of energy fuel sources in all tissues has been shown to occur to favour working muscles (Cherel et al., 1988). A 40% decrease in glucose utilization in the skin has been observed but only at the third and final phase of prolonged starvation, during which a net protein breakdown is occurring (Cherel et al., 1988). This is accompanied by a significant decrease in skin temperature and in peripheral blood circulation (Markussen and Oristland, 1986). GLYCOGEN

METABOLISM

The role of glycogen as an energy store and its biochemical control mechanisms have been well defined in many systems which include muscles and the liver (Newsholme and Leech, 1983). Although all enzymes, that are involved in glycogen synthesis and degradation, have been detected in the epidermis, the exact role of glycogen, in the skin energy metabolism,

1178

D. T. NGUYEN and D. KEAST

has been a controversial issue (Ohkawara et al., 1972). Glycogen which is made up of repeating glucose monomers, has been detected in both the upper and basal epidermal layers of the skin (Zweibaum et al., 1978) and also in hair follicles (Montagna et al., 1951). A significant increase in glucose levels has been shown to correlate with an increase in glycogen concentration in skin in “refeeding” experiments following dietary stress (Harmon and Phizackerley, 1983) and in reperfusion of prolonged ischemic pig skin flaps (Harmon et al., 1986). This suggests that glycogen may act as an energy store in the skin. However, increases in glycogen concentration have also been observed concurrently with high glycolytic activities in damaged or in regenerating skin, such as skin undergoing stripping, wound healing, ultraviolet irradiation or in psoriatic conditions. These observations suggest the presence of a different control mechanism for glycogen synthesis and usage in the skin in these conditions (Lobitz et al., 1962; Im and Hoopes, 1970; Halprin et al., 1973; Vukas et al., 1978; Harmon and Phizackerley, 1984). In addition, glycogen from the skin has also been shown to be used directly by other body systems i.e. muscles in energy demanding situations such as muscular exercise (Pietrzyk and Gjorski, 1980). The pentose phosphate pathway

The phosphogluconate pathway, also known as the pentose phosphate shunt, is a multifunctional pathway which has been shown to be present in tumour cells, lymphocytes and in many other cell types (Kit, 1965; Newsholme et al., 1985). The pentose phosphate pathway has been shown to be active in freshly isolated skin and in cultures of skin (Frienkel, 1960; Bailey, 1971). Increases in the activities of enzymes in this pathway have been observed in psoriatic skin (Halprin and Ohkawara, 1966a), in wound healing skin (Im and Hoopes, 1970) in hyperplastic skin (Foreman et al., 1979) and in growing hair follicles (Adachi and Uno, 1968). The pentose phosphate pathway has been suggested to be responsible for the synthesis of all the carbohydrate requirements for nucleic acids in the skin. The rate of glucose utilization through the pentose phosphate pathway however, is influenced by age (Jongkind et al., 1987). A decrease of more than 50% in the rate of glucose utilization, through the pentose phosphate pathway, has been observed in in vitro cultures of ageing human fibroblasts (Jongkind et al., 1987). The control mechanism for such decreases in the flux of glucose through the pentose phosphate pathway is still unknown even though all enzymes responsible for this pathway have been shown to be fully functional in cultured fibroblasts (Jongkind et al., 1987). The control of glycolysis in the skin has been shown to be influenced by hormonal activities. Epinephrine (Bassukevitz er aI., 1989; Meszaros et al., 1989) and insulin (Ziboh et al., 1971; Lang and Dobrescu, 1989) have been shown to increase both the rate of glycogenolysis and glycolysis in the skin while serotonin (Beitner et al., 1983), bradykinin (Lilling et al., 1983) and vasopressin (Beitner et al., 1984), all have been shown to have the opposite effect.

FATTY ACID METABOLISM

Fatty acids have been shown to generate 39.3 kJ/g in energy in comparison to glucose (15.9 kJ/g) and amino acids (14.6 kJ/g), thus making them a most efficient energy storage type (Ekman and Wretling, 1985). Fatty acids are mobilized, degraded and used as an energy fuel in energy demanding situations such as starvation (Goodman et al., 1980), exposure to cold (Newsholme, 1976), muscular exercise (Costill et al., 1977) and sepsis (Askanazi et al., 1980). Oxidation of fatty acids has been shown to generate Acetyl-CoA and NADH which are essential for the maintenance of the TCA cycle. The role of fatty acids as a fuel source in the skin, has been shown to be only secondary to glucose. This is probably due to the predominance of anaerobic conditions and the abundance of glucose supply in the skin. Nevertheless, the skin has been shown to be able to utilize fatty acids as a fuel for energy, when it was depleted of its primary substrate, glucose, or when there is a strong requirement for an alternative energy source as seen in starvation or in psoriatic conditions (Cruickshank et al., 1962; Cooper et al., 1976). Under these circumstances, a significant increase in the rate of respiration in the skin has been detected when the oxidation of fatty acids occurred (Anastasia and Conley, 1977). Aside from participation of fatty acids as a fuel source in energy demanding situations, in the skin, the fatty acids have also been shown to play a very important role in the preservation of the skin epidermal structure. Lipids, present in the stratum corneum provide the skin with a water permeability barrier. Deficiency in a number of essential fatty acids, such as linoleic acid (18:2), has been shown to cause an impairment of the skin barrier which results in an increase in trans-epidermal water loss and in skin scaliness (Prottey, 1977; Ziboh, 1989). The dermis and the lower basal layer of the skin have been known, for a long time, to be sites for lipogenesis in the skin (Fiengold et al., 1983). Furthermore, the activation of lipogenesis has been shown to be modulated by the skin’s barrier requirement rather than by changes in circulating sterol levels (Menon et al., 1985). Application of topical detergents and acetone to the skin surface, which resulted in trans-epidermal water loss, increased lipogenesis up to three fold in the epidermal layer (Grubauer et al., 1987). However, when trans-epiderma1 loss of water stopped, lipogenesis of the skin returned to normal level, which suggests that changes in transcutaneous water gradients, could be one of the regulatory signals for the lipogenetic activity in the skin (Grubauer et at., 1987). Other regulatory signals could be hormonal. Thyroid hormones have been shown to influence both the concentration and the profile of both serum and cutaneous fatty acids (Campbell and Davis, 1990). The role of fatty acids, as a direct supplier of essential fatty acid precursors in the skin has been neglected. Recent results have provided evidence of a direct uptake, recyling and incorporation of exogenous, essential fatty acids by cultivated keratinocytes into the complex lipids of the stratum corneum layer (Madison et al., 1989; Schurer et al., 1989).

Energy metabolism and the skin GLUTAMINE

METABOLISM

Glutamine has been shown to be the most abundant (20%) amino acid in the blood plasma. It has been shown to act as a storage and transport system for glutamate and for ammonia (NH,) in many tissues and organs (Haussinger and Sies, 1984). Recently, glutamine has also been shown to serve as a transport carrier for cystine, an essential amino acid, in fibroblast cultures, (Bannai and Ishii, 1988). Furthermore, since glutamine has been shown to be a major source of urinary ammonia, it could participate actively in the blood acid-base balance (Meister, 1984). Glutamine is a major nitrogen donor source for the formation of N3 and N9 of the purine and the N’ of the pyrimidine ring (Meister, 1984). In tissue culture, it has been recognized for a considerable time to be a growth limiting amino acid (Eagle, 1955), and absolutely essential for cell replication in cells from a wide variety of sources (Ardawi and Newsholme, 1985; Reitzer et al., 1987; Leighton et al., 1987; Ardawi, 1987; Griffiths and Keast, 1990). It is also proving to be essential for the replication of several viruses in vitro (Goldstein and Guskey, 1984; Glover er al., 1985; Nishio et al., 1990; Vasquez, 1990) and can seriously affect the production of ILI, the first interleukin in the cytokine cascade which leads to the generation of populations of lymphoid cells able to specifically respond to antigenic stimulation (Hebble and Keast, 1991). Glutamine has been shown to be transported from one organ to another by way of blood plasma. It has been shown to be transported across the cell membrane of lymphocytes and other organs using the energy dependent ASC transport system (Felig et al., 1973; Kovacevic and McGivan, 1984; Ardawi and Newsholme, 1986). The ASC transport system which has been shown to be a Na+dependent transport system, can be inhibited competitively by serine or histidine and non-competitively by 2-(methylamino)-isobutyrate and it is thought to be one of the regulating steps involved in the control of glutamine metabolism (Ardawi and Newsholme, 1984, 1986). During the catabolic state, such as in burn injury, intracellular glutamine levels of intestinal and muscle cells have been shown to be reduced, even though the plasma glutamine increases which suggests that the transport system of glutamine was not functioning correctly (Ardawi, 1988; Pietsch et al., 1989). Alterations in the Na+ gradient of the ASC transport system of glutamine, which has been shown to be affected by various hormones and cytokines released during burn injury, could be responsible for the severely reduced levels of glutamine transported across the intestinal or muscle cell membranes (Ardawi, 1988). Furthermore, changes in the concentration of amino acids, i.e. serine and histidine or 2-(methylamino)-isobutyrate, released into the plasma after burning of the skin, have also been suggested to actively compete with glutamine for places in the transport system, therefore reducing the level of glutamine crossing the intestinal and muscle cell membranes (Ardawi, 1988; Pietsch et al., 1989). However, nothing is known of the controlling mechanisms for glutamine transport in skin.

1179

Inter-organ relationship of glutamine synthesis and degradation

Glutamine has been shown to be synthesized from glutamic acid and ammonia, by the enzyme glutamine synthetase. Muscle, along with liver, is now considered to be the most important site for net glutamine synthesis in all species (Lund, 1980). In fact, glutamine and alanine account for nearly 50% of the total amino acids released into the blood after an overnight fast (Lund, 1980). Increases in glutamine released from muscle, have also been observed in starvation (Cahill, 1970), acidosis (Welbourne, 1987) and in trauma, sepsis or burns (Williamson, 1980, Ardawi, 1988). In addition, the rat liver has been shown to release glutamine during acidosis (Welbourne, 1987) starvation or during the consumption of high carbohydrate diets (Remesey et al., 1978). In both the human and the rat, the gut has been shown to be the major site for the uptake of both dietary and circulating glutamine. Up to 66% of dietary glutamine is taken up and used by the gut under normal conditions, while even higher rates of glutamine utilization have been observed during starvation (Windmueller and Spaeth, 1974, 1980). Furthermore, up to 60% of glutamine carbon used in starvation is completely oxidized to CO, which suggests that glutamine and not glucose becomes the main respiratory fuel source for the gut in this condition (Windmueller and Spaeth, 1974, 1978, 1980). Souba and co-workers (1988) have also indicated that during catabolic conditions, the gut undergoes an adaptation which results in a redistribution of the type of fuel used. By switching to glutamine as its main respiratory source, it is possible that the gut could preserve the glucose level in the circulation for tissues that require glucose as their main energy source. Glutamine has also been shown to be essential for the maintenance of the gut integrity. Gut atrophy in the rat has been shown to occur when there is a decrease in glutamine supply (Johnson et al., 1975). Nevertheless, the gut is not the only tissue that uses glutamine. During metabolic acidosis or lactation, the kidney and mammary glands have also been shown to demand an extra supply of glutamine (Lund, 1980). Tumour cells, in vivo, have also been found to actively compete for the body’s own glutamine pool (Rivera et al., 1988) and increases in glutamine utilization have also been observed in wounded tissues (Albina et al., 1987). Recently, significant levels of glutaminase activity have been detected, for the first time in the skin of mice and rats by Keast and co-workers (1989) and Nguyen and Keast (1990a,b) indicating a glutaminolytic capacity for the skin of these animals. The glutaminase activity has been shown to decrease with age and further studies are required to determine the cell types that might lead to a change in glutaminase activity in the skin. Glutaminolysis

Glutamine oxidation in the glutaminolytic pathway has been shown to be mediated by two enzymes: a mitochondrial enzyme L-glutamine amidohydrolase (glutaminase I) and a cytosolic enzyme combination of L-glutamine transaminase and alpha-keto acid

1180

D. T. NGUYEN and D.

W-amidase, known as glutaminase II. The function of the latter enzyme has been suggested to be for the salvage of essential amino acids, by recycling Z-keto acids originally derived from them (Cooper and Meister, 1974). Furthermore, two forms of phosphate dependent and independent glutaminase I exist. The phosphate independent but maleate-stimulated glutaminase catalyses the hydrolysis of glutathione to glutamate and cysteinylglycine while the phosphate dependent form of glutaminase catalyses 90% of the oxidation of glutamine to glutamate (Curthoys et al., 1984; McGivan et al., 1984). The phosphate dependent glutaminase enzyme has been shown to be a mitochondrial enzyme with a molecular weight of 65,000 D in the inactive protomer form. Activation of glutaminase is associated with the dime~zation of the enzyme which then binds readily to glutamine. Increases in glutamate concentration have been shown to reverse this process and to favour the protomer form of glutaminase (Shapiro et al., 1982). In the lymphocyte model, the main end products generated by glutaminolysis are glutamate, aspartate and ammonia (Ardawi and Newsholme, 198.5). The kidney and the small intestine have also been shown to use glutamine through a similar but not identical giutaminolytic pathway (Newsholme and Leech, 1983). Two-oxoglutarate, which is generated by the enzyme aspartate aminotransferase (in lymphocytes), by glutamate dehydrogenase (in kidney) or by alanine aminotransferase (in the small intestine), has been shown to enter into the second half of the TCA cycle via succinyl-CoA as glutamine is used as an energy fuel source (Newsholme and Leech, 1983). A decrease in the level of 2-oxoglutarate has been found to lead to an increase in mitochond~al glutamine transport and glutaminase activities, suggesting that 2-oxoglutarate is one of the controlling substrates in glutaminolysis (Newsholme and Leech, 1983). Furthermore, the generation of 2-oxoglutarate, which enters into the second half of the TCA cycle, could explain why very little acetylCoA is generated from glutamine during glutaminolysis. The fate of pyruvate in glutaminolysis, is still not well understood. It has been suggested that pyruvate is converted back to lactate in the cytosol to further restrict the entry of pyruvate into the TCA cycle (Newsholme and Leech, 1983). The rate of utilization of both glucose and glutamine as fuels has been found to depend on the state and on the type of cell studied. Glucose has been shown to inhibit glutamine oxidation by 85% in cultures of human diploid fibroblasts, while fatty acids or other substrates had no effect (Sumbilla et al., 1981). This has been confirmed by Zielke and co-workers (1984) who also demonstrated that glucose oxidation in cultures of fibroblasts was inhibited by 90% upon the addition of 2mM glutamine. It was, therefore, suggested that fibroblasts met their energy requirement mainly either through anaerobic glycolysis or glutaminolysis. In contrast, lymphocytes in culture have been found to use both glucose and glutamine concurrently as energy sources. Increases in activities of enzymes responsible for glucose and glutamine oxidation have been observed in lymphocyte cultures stimulated antigenically or by both B- and T-cell mitogens (Ardawi and Newsholme, 1983, 1984, 1985; Ardawi,

KEAST

1987; Keast and Newsholme, 1989, 1990; Griffiths and Keast, 1990). However, little information on the role and the dynamics of glutamine metabolism in skin is currently available. Keast et al. (1989) and Nguyen and Keast, (1900a,b) have begun to explore this new area of glutamine metabolism and its dynamics in dietary stress situations.

CONCLUDING

REMARKS

The detection of significant glutaminase activity in the skin along with evidence of its dynamic nature under dietary stress, wound healing and with age (Keast et al., 1989; Nguyen and Keast, 1990a, b) indicates that the energy metabolism of the skin may be more varied than previously thought. It now becomes essential to map the distribution of glutaminase for the cells of the skin and to delineate the significance of glutamine metabolism for the skin and how this may influence glutamine metabolism of the body. Acknowledgement-We acknowledge the financial assistance of Richardson-Vicks Ltd, Villawood, NSW, Australia.

REFERENCES

Adachi K. and Uno H. (1968) Pentose phosphate pathway in growing hair follicles. Am. J. Physiol. 37, 381-386. Aikawa T., Matsutaka H., Yamamoto I-I., Okuda T., Ishikawa E., Kawano T. and Matsumura E. (1973) Gluconeogenesis and amino acid metabolism. II. Interorganal relations and roles of glutamine and alanine in the amino acid metabolism of fasted rats. J. Biochem. 74, 1003-1017. Albina J. E., Henry W., King P. A., Shearer J., MastroFranscesco B., Goldstein L. and Caldwell M. D. (1987) Glutamine metabolism in rat skeletal muscle wounded with Lambda-carrageenan. Am. J. Physiol. 252, E49-E56. Anastasia J. V. and Conley J. P. (1977) The role of fatty acid oxidation in the epidermis. J. invest. Dermatol. 69, 43&434. Ansel J. C., Perry P., Heinrich M., Pham T. and Hefeneider S. (1990) Transcriptional and post transcriptional regulation of IL1 alpha expression in murine keratinocytes by ILI, TNF alpha and GM-CSF (abstr.) C&z. Res. 38, 221A. Ardawi M. S. M. (1987) Glutamine and glucose metabolism in human peripheral lymphocytes. Science Press 7360, 315-317. Ardawi M. S. M. (1988) Skeletal muscle glutamine production in thermally injured rats. Clin. Sci. 74, 165-172. Ardawi M. S. M. and Newsholme E. A. (1983) Glutamine metabolism in lymphocytes of the rat. Biochem. J. 212, 835-842. Ardawi M. S. M. and Newsholme E. A. (1984) Glutamine metabolism in lymphoid tissues. In Glutamine Metabolism in Mammalian Tissues (Edited bv Hgussineer D. and Sies H.) pp. 234-246. Springer, Berljn. Ardawi M. S. M. and Newshohne E. A. (1985) Metabolism in lymph~yt~ and its importance in the immune response. Essays B&hem. 21, l-44. Ardawi M. S. M. and Newsholme E. A. (1986) The transport of glutamine into rat mesenteric lymphocytes. Biochim. biophys. Acta 856, 413420. Arsanow D. M., Kuziel W. A., Bonyhadi M., Tigerlaar R. E., Tucker P. W. and Allison J. P. (1988) Limited diversity of gamma/delta antigen receptor genes of Thy-l + dendritic epidermal cells. Cell 55, 837-845.

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