The interconversion of glycine and serine by plant tissue extracts.

1. Extracts prepared from a variety of higher-plant tissues by ammonium sulphate fractionation were shown to catalyse the interconversion of glycine and serine. This interconversion had an absolute requirement for tetrahydrofolate and appeared to favour serine formation. 2. The biosynthesis of serine from glycine was studied in more detail with protein fractionated from 15-day-old wheat leaves. Synthesis of [(14)C]serine from [(14)C]glycine was not accompanied by labelling of glyoxylate, glycollate or formate. 3. The synthesis of serine from glycine was stimulated by additions of formaldehyde, and [(14)C]formaldehyde was readily incorporated into C-3 of serine in the presence of tetrahydrofolate. 4. The results are interpreted as indicating that serine biosynthesis involves a direct cleavage of glycine whereby the alpha-carbon is transferred via N(5)N(10)-methylenetetrahydrofolate to become the beta-carbon of serine.