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Glycine is converted to carbon dioxide and an intermediate attached to a lipoic acid group on H-protein in the P-protein-catalyzed partial reaction of the glycine cleavage reaction [K. Fujiwara and Y. Motokawa (1983) J. Biol. Chem. 258 , 8156–8162] . The results presented in this paper indicate that the decarboxylation is not accompanied by the removal of a C-2 hydrogen atom of glycine and instead both C-2 hydrogens are transferred with the alpha carbon atom to the intermediate formed during the decarboxylation of glycine. The purified chicken liver cytosolic and mitochondrial serine hydroxymethyltransferase preparations could not catalyze the decarboxylation of glycine in the presence of either lipoic acid or H-protein. The decarboxylation activity of the serine hydroxymethyltransferase preparation purified from bovine liver by the method similar to that of L. R. Zieske and L. Davis [(1983) J. Biol. Chem. 258 , 10355–10359] was completely inhibited by the antibody to P-protein, while the antibody had no effect on the activity of the phenylserine cleavage. Conversely, d -serine inhibited the activity of phenylserine cleavage but the activity of the decarboxylation of glycine was not affected by d -serine. Finally, the two activities were separated by the chromatography on hydroxylapatite. The results clearly demonstrate that serine hydroxymethyltransferase per se cannot catalyze the decarboxylation of glycine.