Time-Course of Changes in Rat Liver Enzyme Activities after Initiation of a High Protein Regimen

Abstract

The effects of feeding a high protein diet to rats previously adapted to a high carbohydrate diet containing adequate protein was studied in male Sprague-Dawley rats. The high carbohydrate diet was found to stimulate growth which was not augmented noticeably by feeding the high protein diet. Liver protein reached maximal values one day after the dietary change, whereas relative liver size values remained relatively constant. The glycogen content of liver was minimum one day after the dietary change which was followed by relatively higher and constant values. The enzymes studied could be divided into 2 groups based on the pattern of change in enzyme activity after the dietary shift. In one group, which includes fructose 1,6-diphosphatase, glucose 6-phosphatase, glutamic-oxaloacetic transaminase, glutamic-pyruvate transaminase, serine dehydrase and tyrosine-α-ketoglutaric transaminase, enzyme activity increased with time. Furthermore, the activity of these enzymes increased without a lag, with the exception of fructose 1,6-diphosphatase, which increased in activity only after a one-day delay. In the second group, which includes L-α-glycerophosphate dehydrogenase, glucose 6-phosphate dehydrogenase, pyruvate kinase and malic enzyme, enzyme activity was decreased by feeding the high protein diet, except that the activities of pyruvate kinase and malic enzyme were temporarily increased one day after the dietary change. The physiological significance of a lag in enzyme induction, when such occurs, and a possible mechanism for the transitory induction of pyruvate kinase and malic enzyme by the high protein diet are discussed.