Regulation of Rat Liver Enzymes by Natural Components of the Diet.

Changes in the metabolism of intact animals caused by reduction or exclusion of carbohydrates from the diet have been the subject of active research (l-3). Changes occurring in the utilization of sugars by isolated tissues of animals fed with different amounts of carbohydrates have also been studied (3-6). The problem of interpreting these facts at the molecular level emerges as an important challenge for those interested in biological adaptation and metabolic regulation. Alterations occurring in the metabolism of a cell undoubtedly reflect modifications in enzyme activity, leading to variations in the relative rates of certain reactions and metabolic sequences and to discrimination among several alternative pathways of metabolism. The occurrence of changes in enzyme activity in mammals brought about by modifications in the quality and quantity of diet seems well established. In particular, the enzymic pattern of liver is substantially modified by changing the nature and amount of food offered to the rat (7,8). Our main efforts have been directed toward obtaining some information on the effect of carbohydrate supply on the activity of liver enzymes directly concerned with carbohydrate utilization. In rats fed a high fat, carbohydrate-free diet, a significant decrease was observed in the activities of several enzymes: glucokinase (EC 2.7.1.2) (9, 10) cr-glucan phosphorylase (EC 2.4.1.1) (1 I), glucose B-phosphate dehydrogenase (EC 1.1.1.49) (9)) phosphogluconate dehydrogenase (decarboxylating) (EC 1.1.1.44)) (9) and uridine diphosphate (UDP) glucosecu-glucan glucosyltransferase (EC 2.4.1.11) (9). Deprivation of food produced a similar effect (9, 12). Analogous results have been obtained by other investigators working under similar conditions (13-15). The dependence of cr-glucan phosphorylase and IJDP-glucosea-glucan glucosyltransferase on dietary carbohydrate has been also evident in animals receiving maltose-dextrin as the sole source of calories; in these animals, values equal to or higher than those of rats fed a balanced diet were observed (9, 11). To our knowledge there is no information on the effect of this particular diet on the other enzymes with which this report is concerned. Dependence on carbohydrates has been further explored by giving glucose after carbohydrate deprivation. It was shown that when a-glucan phosphorylase and glucokinase activities