Role of Enzymes in Metabolic Homeostasis

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Abstract
The dietary depletion and restoration of avian hepatic enzymes involved in carbohydrate metabolism were studied. Enzyme activities were depleted by 7-day fasting and restoration was induced by a one-day refeeding period. In chickens 7-day fasting decreased glucose-6-phosphate dehydrogenase activity to zero, lactic dehydrogenase to 64%, and fructose-1, 6-diphosphatase, 6-phosphogluconate dehydrogenase, phosphoglucomutase and phosphohexoseisomerase activities to 39 to 48% of normal fed values. On refeeding, glucose-6-phosphate dehydrogenase increased to 300% and all other enzyme activities showed increases of 21 to 51%. These results show that the various enzymes involved in glucose-6-phosphate metabolism show widely different behavior during nutritional depletion and restoration processes and do not all simultaneously change with the altering substrate level. Thus, the behavior of avian hepatic carbohydrate-metabolizing enzymes basically resembles that of mammals under the same conditions. These observations are well in line with the presented data which demonstrated that the general pattern of absolute activities of avian and rodent liver carbohydrate-metabolizing enzymes is also similar.