A comparison of alanine and glucose as precursors of serine and glycine

Abstract

Young rats were fed on synthetic diets containing 0.59% of L-[beta-Cl4] alanine, 0.5% of uniformly labeled L-[C14] alanine 34% of D-[1-Cl4] -glucose or 34% of uniformly labeled D-(C14) glucose for periods of 13-20 days under defined conditions in the presence and absence of dietary pteroylglutamic acid and vitamin B12. At the end of the experimental feeding period, alanine, aspartic acid, glutamic acid, serine and glycine were isolated from the protein of the mixed internal organs and their radioactivity was determined. When uniformly labeled glucose was fed all the isolated amino acids had similar radioactivities. Degradation of the serine and glycine showed that the distribution of C14 in these amino acids was essentially uniform. [1-C14] Glucose gave rise to similar radioactivities in the isolated alanine and serine; the serine contained 55-65% of the total radioactivity in the beta-carbon atom. When labeled alanine was fed serine and glycine were much less radioactive than the isolated alanine, aspartic acid and glutamic acid. Deficiency of pteroylglutamic acid, but not of vitamin B12, specifically reduced the incorporation of radioactivity from labeled alanine or glucose into glycine, while it had no effect on the radioactivity of serine. It is concluded that serine is an important intermediate in glycine biosynthesis but that serine biosynthesis from non-nitrogenous sources does not require pteroylglutamic acid or the prior formation of glycine. Serine is synthesized from a non-nitrogenous C3 precursor which is more closely related to an intermediate of the glycolytic pathway of glucose catabolism, possibly a triose or glyceric acid, than to pyruvate. The conversion of glucose, labeled in C(1) or in ail carbon atoms, into alanine was used to calculate the relative contribution of the glycolytic and oxidative pathways of glucose catabolism. In the intact rat none of the pyruvate appears to arise by the oxidative catabolism of glucose.