898

BIOCHEMICAL SOCIETY TRANSACTIONS

06

0.8

1.0

[NADP+I/[NADPH] Fig. 1, Calculated variation of relative initial rates for isocitrate dehydrogenase The reaction curves were calculated according to eqn. (I), using values of K, = K , , = 3 . 5 , u ~ ,K b = 3 . 4 p ~ K,, , = 9 . 2 and ~ ~ Kiq= 9 . 7 ~ The ~ . total NADP+NADPH con, was measured by the recycling method of Lowry & Passonneau centration was 6 5 p ~and (1972). D-isocitrate concentrations are indicated on the curves and the shaded area represents the physiological range of D-isocitrate concentrations in the digestive gland of M . edulis. D-Isocitrate concentrations have been determined in individual mussels. I n the digestive gland these lie between the values of K,,, and about 10 K,, with most values in the ~ . NADP+-dependent cytoplasmic dehydrogenases, Sols & Marco range 2 0 - 4 0 ~ For (1970) consider that any utilization of NADPH by biosynthetic pathways will be followed by a fall in N A D P H concentration and a n increased regeneration of NADPH. At saturating concentrations of D-isocitrate and in the presence of 5 m ~ - M g ' + ,isocitrate dehydrogenase behaves in accordance with this prediction. Addink, A. D. F. & Veenhof, P. R. (1975) in Proc. Eur. Mar. Biol. Symp. 9th (Barnes, H., ed.), pp. 109-1 19, Aberdeen University Press, Aberdeen Atkinson, D. E. (1970) Enzymes, 3rd Ed. 1,461-489 Bailey, E. (1975) in Insect Biochemistry and Function, (Candy, D. J. & Kilby, B. A,, eds.), pp. 89-167, Chapman and Hall, London Bayne, B. L. (1973) Neth. J. Sea Res. 7, 399410 Carrion-Angosto, A., Silva Pando, M., Rodrigues-Segade, S. & Ruiz-Amil, M. (1977) Rev. ESP. Fisiol. 33,265-272 De Zwaan, A. &Van Marrewijk, W. J. A. (1973) Comp. Biochem. Physiot. B44,1057-1066 Fromm, H. J. (1975) Initial Rate Kinetics, Springer-Verlag, Berlin, Heidelberg and New York Gul, B. & Dils, R. (1969) Biochem. J. 112,293-301 Lowry, 0. H. & Passonneau, J. V. (1972) A Flexible System of Enzymatic Analysis, Academic Press, New York and London Rodriguez-Segade, S., Freire, M. & Carrion, A. (1978) Biochem. J. 170, 577-582 Sols, A. & Marco, R. (1970) Curr. Top. Cell. Regul. 2,227-273 Uhr, M. L., Thompson, V. W. & Cleland, W. W. (1974) J. Biol. Chem. 249,2920-2927

Seasonal Changes in the Triacylglycerol Fatty Acids of the Mantle Tissue of the Mussel Mytifus edufis L. MICHAEL J. WALDOCK and DAVID L. HOLLAND NERC Unit of Marine Invertebrate Biology, Marine Science Laboratories, Menai Bridge, Gwynedd LL59 5EH, Wales, U.K.

It has been shown in this laboratory that juvenile bivalves have a limited capacity for elongation and desaturation of fatty acids and that for optimal growth certain fatty acids

1979

583rd MEETING, CAMBRIDGE

899

must be provided in the diet (M. J. Waldock & C. J. Langdon, unpublished work). A requirement for essential fatty acids has been shown for prawns (Kanazawa et al., 1977a,b) and is well established for fish and higher vertebrates. Therefore the essential fatty acids found in the triacylglycerol reserves of bivalve eggs must have been incorporated into the eggs directly from the diet. However, there is also evidence to indicate that fatty acids of the egg triacylglycerols are synthesized de nouo during the winter within the mantle tissue, from existing large glycogen stores as discussed by Gabbott (1975). In addition, B. N. Zaba (personal communication) has shown that in uitro I4C from [14C]glucosecan be incorporated into the triacylglycerol fraction of Mytilus edulis mantle during gametogenesis. Fig. 1 shows the content of triacylglycerol and the triacylglycerol saturated, monounsaturated and polyunsaturated fatty acids in the mantle of M . edulis during 1976-1978. Although there were marked fluctuations in the content of triacylglycerol, the relative proportions of the fatty acid groups remained similar until the winter of 1977, when saturated and particularly the monounsaturated fatty acids of the mantle triacylglycerol increased relative to the polyunsaturated fatty acids. The increase in the monounsaturated fatty acids was due largely to an increase in 16 : I (w7), palmitoleic acid. Seasonal increases in the concentration of palmitoleic acid have been shown in a number of other bivalve species (Calzolari et al., 1971). Palmitoleic acid, can be synthesized in animal systems by a A9-'O-desaturase from palmitic acid (see Jeffcoat et al., 1977), and a high-carbohydrate diet has been shown to elevate both the activity of fatty acid synthetase and A9-'O-desaturase (Jeffcoat & James, 1977). Therefore the presence of a large amount of palmitoleic acid suggests that in the winter of 1977 there was significant synthesis de nouo of triacylglycerol fatty acids from glycogen. In Fig. 1 the broken line indicates the calculated triacylglycerol content of the mantle in the winter of 1977-1978 if the proportions of the saturated and monounsaturated fatty acids had remained

1976

.,

1977

1978

Fig. 1. Seasonal changes in the content of triacylglycerol and triacylglycerol fatty acids in the mantle tissue of M . edulis Triacylglycerol; A, polyunsaturated fatty acids; a, monounsaturated fatty acids; fatty acids.

0,saturated

VOl. 7

BIOCHEMICAL SOCIETY TRANSACTIONS

900 Exogenous

\

I I I

Endogenous

I

I

q ,gen

1

lipid

Fig. 2. Sources of egg lipid

constant. The increase in triacylglycerol content is represented by the shaded area and accounts for up to 25 % of triacylglycerol fatty acids present during gametogenesis. It is proposed that there is a balance between exogenous (dietary) and endogenous (from mantle glycogen) sources of egg lipid, which depends on the amount of food available during each season. This is shown in Fig. 2. Throughout 1976-1977 the dietary uptake of lipid may have been sufficient or very nearly sufficient to provide all the fatty acids for incorporation into egg triacylglycerol. In the winter of 1977 there may have been less lipid available, and the mantle triacylglycerol, which decreased during November-December, was restored to the normal concentration by an increase in synthesis de novo of fatty acids, particularly palmitoleic acid, from the stored glycogen reserves. In this way the mussel is able to buffer against changes in food availability, and maintain a high concentration of mantle triacylglycerol during the period of gametogenesis. Calzolari, C., Cerrna, E. & Stancher, 8.(1971) La Riuisfa Iral. Sostanze Grasse 48rh, 605-611 Gabbott, P. A. (1975)Proc. Eur. Mar. Biol. Symp. 9th (Barnes, H., ed.), pp. 191-211, Aberdeen University Press, Aberdeen Jeffcoat, R., Brown, P. R., Safford, R. &James, A. T. (1977) Biochern. J . 161,431-437 Jeffcoat, R. & James, A. T. (1977) Lipids 12,469-474 Kanazawa, A,, Teshirna, S. & Tokiwa, S. (1977~)Bull. Jpn. SOC.Sci. Fish.43, 849-856 Kanazawa, A , , Tokiwa, S., Kayarna, M. & Hirata, M. (1977b) Bull. Jpn. Soc. Sci. Fish. 43, 1111-1114

The Contribution of the Pentose Phosphate Cycle to the Central Pathways of Metabolism in the Marine Mussel, Mytifus edufis L. BOGUMIL N. ZABAand J. ISLWYN DAVIES

Department of Biochemistry and Soil Science, University College of North Wales, Bangor, Gwynedd LL57 2UW, Wales, U.K. It is clear that the metabolism of the marine mussel has undergone extensive adaptation to its inter-tidal environment, in which there occur both daily fluctuations in oxygen 1979