sac can be used by the fetus, much more work is needed before the Drocedure is adoDted. Among the factors which need consideration are the danger of amniocentesis itself, together with the fact that large amounts of some amino acids may actually impede growth and mental and motor development. 0
3.
4.
5.
1. D. Gitlin, J. Kumate, C. Morales, L. Noriegan
6.
and N. Arevalo: The Turnover of Amniotic Fluid Protein in the Human Conceptus. Am. J . Obstet. Gynec. 113: 632-645, 1972 2. F. Cozzi and A.W. Wilkinson: Intrauterine Growth Rate in Relation to Anorectal and
7.
Oesophageal Anomalies. Arch. Dis. Child. 44: 59-62, 1969 R.M. Pitkin and W.A. Reynolds: Fetal Ingestion and Metabolism of Amniotic Fluid Protein. Am. J. Obstet. Gynec. 123: 356-363, 1975 J.A. Pritchard: Deglutition by Normal and Anencephalic Fetuses. Obstet. Gynecol. 25: 289-297, 1965 L.R. da Costa: Protein Loss from the Normal Human Small Intestinal Mucosa. J. Nutrition 101: 431-436, 1971 R. Lev and D. Orlic: Protein Absorption by the Intestine of the Fetal Rat In Utero. Science 177: 522-524, 1972 K. Baintner, Jr.: Possible Protein-Absorptive Ability in Human Fetuses. Gastroenterology 65: 695-697, 1973
ADAPTIVE RESPONSES OF AMINO ACID DEGRADING ENZYMES TO VARIATION OF AMINO ACID AND PROTEIN INTAKE In correspondence to its ability to maintain weight, the rat shows in lysine deficiency but not in threonine deficiency an adaptive decrease in the hepatic enzyme primarily concerned with breakdown of the amino acid.
Key Words: indispensable amino acids, lysine, threonine, threonine dehydratase, lysine-ketoglutarate reductase, enzyme induction, proteinfree diets, high-protein diets
A classical assumption due to Block and Mitchell' holds that the absence of any one of the essential amino acids in the diet should limit protein synthesis to a similar degree, representing in each case the nutritional equivalent of a protein-free diet. Experiments in recent years indicated, however, that animals tolerate a restricted intake of some essential amino acids more than of others. These differences are attributed to adaptive slowing of the degradation of the amino acid as it comes to be in short supply. Said and Hegsted,Z and Said, Hegsted and Hayes3 showed that adult rats make little adaptation of this kind when fed a threonine-free diet, but a relatively large adaptation to a lysine-free diet, with the result that weight is lost more slowly on the latter. The major enzymes initiating the breakdown of these two amino acids in the rat are not the
usual range of transaminases but instead two special enzymes, namely threonine (or serine) dehydratase and lysine-ketoglutarate reductase, the latter enzyme leading to theformation of saccharopine. A special opportunity is therefore provided to contrast the responsiveness of these enzymes to deficiencies of their respective substrates. Chu and Hegsted4have now studied in detail the adaptive responses of these two enzymes in the adult rat. Charles River female rats of approximately 200 g were used. As an apparently straightforward answer to the question posed above, these authors found the activity of the lysine-degrading enzyme in the liver was significantly diminished (to about onethird) during four weeks on a lysine-free diet containing 3.25 percent of an amino acid mixture. Such a decrease was not obtained on a diet free of both proteins and amino acids. In contrast the omission of threonine from an otherwise complete amino acid mixture fed over the same interval did not significantly change the hepatic threonine dehydratase activity. NUTRITION REVIEWSNOL. 34, NO. 11INOVEMBER 1976 343
The much larger fall of the activity of the relevant enzyme on a lysine-free diet than on a protein-free diet was rationalized as arising from the larger endogenous release of lysine on the latter diet. Plasma lysine had earlier been shown to remain much higher on the latter than on the former regimen. The results are more complicated, however, when the increases in the activity of these two hepatic enzymes i n response to dietary changes are considered. To begin with, the animals on the stock diet showed hepatic activities for threonine dehydratase nearly 20 times as high as those observed on the compiete amino acid diet, on a protein-free diet or on a 5 or 10 percent lactalbumin diet. The activity of this enzyme in the liver was increased sharply in passing from 20 to 30 percent protein in the experimental diet (three weeks feeding), whether lactalbumin or wheat gluten was used. The activity came to exceed that on the stock diet when wheat gluten was brought to 40 or 50 percent. The activity of the enzyme was not increased significantly, however, on adding 4 percent threonine to the 5 percent lactalbumin diet. Harper had found earlier that hepatic threonine dehydratase activity was increased rapidly only at protein intakes higher than those required for growth;s Goldstein, Knox and Behrman found that threonine feeding does not increase hepatic threonine dehydratase activity.6 The hepatic activity of lysine-ketoglutarate reductase was also increased by feeding highprotein diets, an effect that was already apparent at 20 percent lactalbumin, but only at 40 percent wheat gluten. The latter protein is well known to be a poor source of lysine. The addition of 2 percent lysine hydrochloride to the 5 percent lactalbumin diet approximately tripled the activity of the hepatic lysine-degrading enzyme in two weeks. These results correspond to a substrateinduced increase in enzyme activity in the case of lysine deficiency, although the nature of the dietary protein affected the results in ways that were only partially explained by differences in dietary lysine content. In contrast, the changes in hepatic threonine dehydratase do not correspond to a substrate induction. Prior research by Mauron, Mottu and Spohr7 has pointed to other amino acids as possibly in344 NUTRITION REVIEWSIVOL. 34. NO. 1lINOVEMEER 1976
volved in the stimulus to elevation of the activity of this enzyme on high-protein diets. Increases in degradative capacity on high intake of amino acids or protein have had prior study and were reviewed in 1970 by Harper, Benevenga and Woblhuetter.8 Such adaptive responses have obvious value in protecting the organism from high accumulations of amino acids, not an insignificant risk even though diets containing 40 or 50 percent protein are largely a laboratory curiosity. In contrast, conservative responses to amino acid deficiency of the type shown by the enzyme principally initiating lysine degradation answer to a different biological hazard, and are perhaps not likely to be mechanistically the same or to lie in any direct parallelism to the conservative responses. We may therefore regard as important the correspondence found in the present study between the tolerance for lysine deficiency, relative to threonine deficiency, and the conservative decrease in lysine degrading activity, not seen for threonine degrading activity. The circumsrance that the behavior of these enzymes at elevated protein intakes represents more complicated aspects of regulation, especially in the case of the dehydratase, does not seem to decrease the significance of this correspondence in the important approach to explanations for the relative conservation of amino acids in short supPly. The risk of lysine deficiency has undoubtedly occurred more regularly during the development of herbivorous and omnivorous animals than for carnivorous species. It is likely but not certain that man will prove to share with the rat this special conservation of lysine when this amino acid is in short supply. 0
1. R.J. Block and H.H. Mitchell: The Correlation of the Amino Acid Composition of Proteins with their Nutritive Value. Nutrition Abst. Rev. 16: 249-278, 1946-47 2. A.K. Said and D.M. Hegsted: Response of Adult Rats to Low Dietary Levels of Essential Amino Acids. J. Nutrition 100: 1363-1376, 1970 3. A.K. Said, D.M. Hegsted and K.C. Kayes: Response of Adult Rats to Deficiencies of Different Essential Amino Acids. Brit. J. Nutrition 31: 47-57, 1974
S.-H. W. Chu and D.M. Hegsted: Adaptive Response of Lysine and Threonine Degrading Enzymes in Adult Rats. J . Nutrition 106: 10891096, 1976 A.E. Harper: Diet and Plasma Amino Acids. Am. J. Clin. Nutrition. 21: 358-366. 1968 L. Goldstein, W.E. Knox and E.J. Behrman: Studies on the Nature, Inducibility and Assay of the Threonine and Serine Dehydratase Activities in Rat Liver. J. Biol. Chem. 237: 2855-2860, 1962
7. J. Mauron, F. Mottu and G. Spohr: Reciprocal Induction and Repression of Serine Dehydratase and Phosphoglycerate Dehydrogenase by Proteins and Dietary-Essential Amino Acids in Rat Liver. Eur. J . Biochem. 23: 331-342, 1973 8. A.E. Harper, N.J. Benevenga and R.M. Wohlhuetter: Effects of Ingestion of Disproportionate Amounts of Amino Acids. Physiol. Rev. 50: 428558, 1970
STIMULUS FOR HYPERPLASIA OF THE SMALL BOWEL When large portions of the small intestine are resected, oral intake of nutrients is necessary for hyperplasia of the remaining small intestine to occur.
Key Words: intestine, resection, hyperplasia, nutrients
Compensatory hypertrophy of remaining gut usually occurs when portions of the small intestine are removed. For instance, humans with resection of large lengths of small intestine due to Crohn’s disease, intestinal obstruction or other causes show dilatation of the remaining gut not seen in the normal state. After gastric surgery when, on occasions, the jejunum is approximated to the stomach, dilatation of the jejunum occurs. It has been reasoned that this is due to the delivery of nutrients directly to an area of jejunum, which is normally not directly exposed to gastric contents. It has been assumed that it is the nature of gastric contents which produces the dilatation. Sometimes, such dilatation is mistakenly confused with pathologic damage. This type of dilatation, however, appears to be a physiologic response on the part of the intestine due to loss in length which increases the surface area for absorption. The stimulus for hyperplasia is poorly understood. It has been assumed to be due to either direct contact of dietary nutrients in forms not usually presented to the small bowel mucosa or via hormonal or neurovascular factors initiated through nutrient contact. A study was designed to test whether dietary factors are nec-
essary for the development of hyperplasia in the remaining small intestine after resection of large portions of small intestine.’ In rats, 70 cm of proximal small intestine beginning 5 cm distal to the ligament of Treitz and 35 cm proximal to the ileal cecal junction were resected. Control animals were sham operated. Six animals were given oral alimentation with an elemental diet consisting of 30 percent dextrose, 5 percent amino acids, electrolytes and vitamins. A group of seven resected and six sham-operated animals were given intravenous alimentation with the same elemental diet, receiving about 45 to 50 ml per day by continuous infusion. The orally fed animals with small intestinal resection lost about 15 g of weight during one week. Average daily energy intake was between 35 to 40 calories. The intravenously alimentated animals maintained their body weight. In the animals who lost the small intestine and who were fed by mouth, significant small bowel hyperplasia, proximal and distal to the anastomoses, occurred. Hyperplasia was more marked, however, in the segment of gut distal to the anastomoses. Gut and mucosal weight, mucosal protein and DNA content in each segment of gut studied from these animals were significantly greater than comparable segments in the sham-operated, orally fed animals. NUTRITION REVIEWSIVOL. 34, NO. 11INOVEMBER 1976 345
3.
4.
5.
1. D. Gitlin, J. Kumate, C. Morales, L. Noriegan
6.
and N. Arevalo: The Turnover of Amniotic Fluid Protein in the Human Conceptus. Am. J . Obstet. Gynec. 113: 632-645, 1972 2. F. Cozzi and A.W. Wilkinson: Intrauterine Growth Rate in Relation to Anorectal and
7.
Oesophageal Anomalies. Arch. Dis. Child. 44: 59-62, 1969 R.M. Pitkin and W.A. Reynolds: Fetal Ingestion and Metabolism of Amniotic Fluid Protein. Am. J. Obstet. Gynec. 123: 356-363, 1975 J.A. Pritchard: Deglutition by Normal and Anencephalic Fetuses. Obstet. Gynecol. 25: 289-297, 1965 L.R. da Costa: Protein Loss from the Normal Human Small Intestinal Mucosa. J. Nutrition 101: 431-436, 1971 R. Lev and D. Orlic: Protein Absorption by the Intestine of the Fetal Rat In Utero. Science 177: 522-524, 1972 K. Baintner, Jr.: Possible Protein-Absorptive Ability in Human Fetuses. Gastroenterology 65: 695-697, 1973
ADAPTIVE RESPONSES OF AMINO ACID DEGRADING ENZYMES TO VARIATION OF AMINO ACID AND PROTEIN INTAKE In correspondence to its ability to maintain weight, the rat shows in lysine deficiency but not in threonine deficiency an adaptive decrease in the hepatic enzyme primarily concerned with breakdown of the amino acid.
Key Words: indispensable amino acids, lysine, threonine, threonine dehydratase, lysine-ketoglutarate reductase, enzyme induction, proteinfree diets, high-protein diets
A classical assumption due to Block and Mitchell' holds that the absence of any one of the essential amino acids in the diet should limit protein synthesis to a similar degree, representing in each case the nutritional equivalent of a protein-free diet. Experiments in recent years indicated, however, that animals tolerate a restricted intake of some essential amino acids more than of others. These differences are attributed to adaptive slowing of the degradation of the amino acid as it comes to be in short supply. Said and Hegsted,Z and Said, Hegsted and Hayes3 showed that adult rats make little adaptation of this kind when fed a threonine-free diet, but a relatively large adaptation to a lysine-free diet, with the result that weight is lost more slowly on the latter. The major enzymes initiating the breakdown of these two amino acids in the rat are not the
usual range of transaminases but instead two special enzymes, namely threonine (or serine) dehydratase and lysine-ketoglutarate reductase, the latter enzyme leading to theformation of saccharopine. A special opportunity is therefore provided to contrast the responsiveness of these enzymes to deficiencies of their respective substrates. Chu and Hegsted4have now studied in detail the adaptive responses of these two enzymes in the adult rat. Charles River female rats of approximately 200 g were used. As an apparently straightforward answer to the question posed above, these authors found the activity of the lysine-degrading enzyme in the liver was significantly diminished (to about onethird) during four weeks on a lysine-free diet containing 3.25 percent of an amino acid mixture. Such a decrease was not obtained on a diet free of both proteins and amino acids. In contrast the omission of threonine from an otherwise complete amino acid mixture fed over the same interval did not significantly change the hepatic threonine dehydratase activity. NUTRITION REVIEWSNOL. 34, NO. 11INOVEMBER 1976 343
The much larger fall of the activity of the relevant enzyme on a lysine-free diet than on a protein-free diet was rationalized as arising from the larger endogenous release of lysine on the latter diet. Plasma lysine had earlier been shown to remain much higher on the latter than on the former regimen. The results are more complicated, however, when the increases in the activity of these two hepatic enzymes i n response to dietary changes are considered. To begin with, the animals on the stock diet showed hepatic activities for threonine dehydratase nearly 20 times as high as those observed on the compiete amino acid diet, on a protein-free diet or on a 5 or 10 percent lactalbumin diet. The activity of this enzyme in the liver was increased sharply in passing from 20 to 30 percent protein in the experimental diet (three weeks feeding), whether lactalbumin or wheat gluten was used. The activity came to exceed that on the stock diet when wheat gluten was brought to 40 or 50 percent. The activity of the enzyme was not increased significantly, however, on adding 4 percent threonine to the 5 percent lactalbumin diet. Harper had found earlier that hepatic threonine dehydratase activity was increased rapidly only at protein intakes higher than those required for growth;s Goldstein, Knox and Behrman found that threonine feeding does not increase hepatic threonine dehydratase activity.6 The hepatic activity of lysine-ketoglutarate reductase was also increased by feeding highprotein diets, an effect that was already apparent at 20 percent lactalbumin, but only at 40 percent wheat gluten. The latter protein is well known to be a poor source of lysine. The addition of 2 percent lysine hydrochloride to the 5 percent lactalbumin diet approximately tripled the activity of the hepatic lysine-degrading enzyme in two weeks. These results correspond to a substrateinduced increase in enzyme activity in the case of lysine deficiency, although the nature of the dietary protein affected the results in ways that were only partially explained by differences in dietary lysine content. In contrast, the changes in hepatic threonine dehydratase do not correspond to a substrate induction. Prior research by Mauron, Mottu and Spohr7 has pointed to other amino acids as possibly in344 NUTRITION REVIEWSIVOL. 34. NO. 1lINOVEMEER 1976
volved in the stimulus to elevation of the activity of this enzyme on high-protein diets. Increases in degradative capacity on high intake of amino acids or protein have had prior study and were reviewed in 1970 by Harper, Benevenga and Woblhuetter.8 Such adaptive responses have obvious value in protecting the organism from high accumulations of amino acids, not an insignificant risk even though diets containing 40 or 50 percent protein are largely a laboratory curiosity. In contrast, conservative responses to amino acid deficiency of the type shown by the enzyme principally initiating lysine degradation answer to a different biological hazard, and are perhaps not likely to be mechanistically the same or to lie in any direct parallelism to the conservative responses. We may therefore regard as important the correspondence found in the present study between the tolerance for lysine deficiency, relative to threonine deficiency, and the conservative decrease in lysine degrading activity, not seen for threonine degrading activity. The circumsrance that the behavior of these enzymes at elevated protein intakes represents more complicated aspects of regulation, especially in the case of the dehydratase, does not seem to decrease the significance of this correspondence in the important approach to explanations for the relative conservation of amino acids in short supPly. The risk of lysine deficiency has undoubtedly occurred more regularly during the development of herbivorous and omnivorous animals than for carnivorous species. It is likely but not certain that man will prove to share with the rat this special conservation of lysine when this amino acid is in short supply. 0
1. R.J. Block and H.H. Mitchell: The Correlation of the Amino Acid Composition of Proteins with their Nutritive Value. Nutrition Abst. Rev. 16: 249-278, 1946-47 2. A.K. Said and D.M. Hegsted: Response of Adult Rats to Low Dietary Levels of Essential Amino Acids. J. Nutrition 100: 1363-1376, 1970 3. A.K. Said, D.M. Hegsted and K.C. Kayes: Response of Adult Rats to Deficiencies of Different Essential Amino Acids. Brit. J. Nutrition 31: 47-57, 1974
S.-H. W. Chu and D.M. Hegsted: Adaptive Response of Lysine and Threonine Degrading Enzymes in Adult Rats. J . Nutrition 106: 10891096, 1976 A.E. Harper: Diet and Plasma Amino Acids. Am. J. Clin. Nutrition. 21: 358-366. 1968 L. Goldstein, W.E. Knox and E.J. Behrman: Studies on the Nature, Inducibility and Assay of the Threonine and Serine Dehydratase Activities in Rat Liver. J. Biol. Chem. 237: 2855-2860, 1962
7. J. Mauron, F. Mottu and G. Spohr: Reciprocal Induction and Repression of Serine Dehydratase and Phosphoglycerate Dehydrogenase by Proteins and Dietary-Essential Amino Acids in Rat Liver. Eur. J . Biochem. 23: 331-342, 1973 8. A.E. Harper, N.J. Benevenga and R.M. Wohlhuetter: Effects of Ingestion of Disproportionate Amounts of Amino Acids. Physiol. Rev. 50: 428558, 1970
STIMULUS FOR HYPERPLASIA OF THE SMALL BOWEL When large portions of the small intestine are resected, oral intake of nutrients is necessary for hyperplasia of the remaining small intestine to occur.
Key Words: intestine, resection, hyperplasia, nutrients
Compensatory hypertrophy of remaining gut usually occurs when portions of the small intestine are removed. For instance, humans with resection of large lengths of small intestine due to Crohn’s disease, intestinal obstruction or other causes show dilatation of the remaining gut not seen in the normal state. After gastric surgery when, on occasions, the jejunum is approximated to the stomach, dilatation of the jejunum occurs. It has been reasoned that this is due to the delivery of nutrients directly to an area of jejunum, which is normally not directly exposed to gastric contents. It has been assumed that it is the nature of gastric contents which produces the dilatation. Sometimes, such dilatation is mistakenly confused with pathologic damage. This type of dilatation, however, appears to be a physiologic response on the part of the intestine due to loss in length which increases the surface area for absorption. The stimulus for hyperplasia is poorly understood. It has been assumed to be due to either direct contact of dietary nutrients in forms not usually presented to the small bowel mucosa or via hormonal or neurovascular factors initiated through nutrient contact. A study was designed to test whether dietary factors are nec-
essary for the development of hyperplasia in the remaining small intestine after resection of large portions of small intestine.’ In rats, 70 cm of proximal small intestine beginning 5 cm distal to the ligament of Treitz and 35 cm proximal to the ileal cecal junction were resected. Control animals were sham operated. Six animals were given oral alimentation with an elemental diet consisting of 30 percent dextrose, 5 percent amino acids, electrolytes and vitamins. A group of seven resected and six sham-operated animals were given intravenous alimentation with the same elemental diet, receiving about 45 to 50 ml per day by continuous infusion. The orally fed animals with small intestinal resection lost about 15 g of weight during one week. Average daily energy intake was between 35 to 40 calories. The intravenously alimentated animals maintained their body weight. In the animals who lost the small intestine and who were fed by mouth, significant small bowel hyperplasia, proximal and distal to the anastomoses, occurred. Hyperplasia was more marked, however, in the segment of gut distal to the anastomoses. Gut and mucosal weight, mucosal protein and DNA content in each segment of gut studied from these animals were significantly greater than comparable segments in the sham-operated, orally fed animals. NUTRITION REVIEWSIVOL. 34, NO. 11INOVEMBER 1976 345
