Comparative Studies on the Pathways for Serine Biosynthesis in Animal Tissues

The pathways of serine biosynthesis utilizing either phosphorylated or nonphosphorylated intermediates have been investigated in a variety of tissues from different species of animals. The levels of activity of hydroxypyruvate phosphate:l-glutamate and hydroxypyruvate:l-alanine transaminases were determined, and their distribution was compared with that previously reported for d-glycerate 3-O-phosphate and d-glycerate dehydrogenases. Direct comparisons were made between the extent of the over-all conversion of d-glycerate to serine and of d-glycerate 3-O-phosphate to serine 3-O-phosphate catalyzed by liver preparations from six species. Additional studies were carried out on the specificity of the amino donor participating in transamination reactions with hydroxypyruvate in different tissues. The results indicate that although both pathways are functional in animal systems, the relative contribution by each pathway for the synthesis of serine is not the same for all tissues. For example, in dog and frog liver, the major pathway for the formation of this amino acid involves nonphosphorylated intermediates. In these two tissues, as well as in rabbit liver, it has been clearly shown that the amino donor for hydroxypyruvate transamination is alanine. Glutamine has been demonstrated to be the most effective amino donor for this reaction in rat kidney and liver. In contrast to these results, serine formation in beef and chicken liver has been shown to take place primarily by the phosphorylated pathway. In other tissues, e.g. pig liver and dog kidney, considerable serine biosynthesis occurs by both the phosphorylated and nonphosphorylated routes.