Serine hydroxymethylase. Specificity of bond cleaveage to form quinonoid intermediates and rate of holoenzyme formation.

L-Serine transhydroxymethylase (5,10-methylenetetrahydrofolate:glycine hydroxymethyltransferase, EC 2.1.2.1) a pyridoxal phosphate-dependent enzyme, has been obtained as a homogeneous preparation with a specific activity of 6.7 mumol benzaldehyde per minute at 30 degrees C at pH 7.5 in N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (Hepes) buffer, with DL-threo-beta-phenylserine as a substrate. This enzyme has been used to study the specificity of bond cleavage in forming quinonoid intermediates from DL and non-asymmetric amino acids. The ability of the generated quinonoids to react with formaldehyde and acetaldehyde has also been studied and evidence obtained for formation of the corresponding beta-hydroxymethyl and beta-hydroxyethyl amino acid derivaties. Apotranshydroxymethylase has been prepared and the rate of holoenzyme formation was found to be 0.52 min-1 by measuring Schiff base formation at 425 nm and 0.66 min-1 as determined from restoration of enzymic activity. A requirement for the presence of mercaptoethanol for complete reactivation was also established by these studies.