Dietary and hormonal regulation of serine synthesis in the rat.

The importance of the so-called phosphorylated pathway for the synthesis of serine in rat liver was confirmed by studies on in vitro serine synthesis and also by the existence of a correlation between the serine-synthesizing activity of liver extracts and the activities of the enzymes involved in the pathway under various dietary conditions. Serine-synthesizing activity was found to be distributed in various rat tissues such as kidney, brain, testis, spleen, pancreas, and fat pad. However, only in the liver was the synthesis regulated by dietary protein. In the liver, the three enzymes of the phosphorylated pathway were found to be repressed by high-protein diets or by starvation and induced by low-protein diets. The dietary induction of the enzymes required the presence of insulin and was suppressed by glucocorticoids. A suggestion is made that the effects of diet or hormones may be mediated by changes in the hepatic pool of essential amino acids.