The interconversion of serine and glycine: participation of pyridoxal phosphate

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Abstract
A manometric method is presented for the estimation of the activity of the enzyme system synthesizing serine from glycine and formaldehyde. The enzyme system was partially purified by heat treatment, ammonium sulfate fractiona-tion and ethanol fractionation. During enzyme fractionation the requirement for pyridoxal phosphate increased. A twofold increase in the activity of the partially purified enzyme was produced by addition of 5 x 10-4 [image] pyridoxal phosphate to the reaction mixture. Enzyme incubated at about pH 9 and 37[degree] for 15-30 minutes was largely inactivated, but a substantial proportion of the activity could be recovered by incubating the enzyme with pyridoxal phosphate at pH 7.2 for a short period. Half the maximum reactivation was produced by 10-6 [image] pyridoxal phosphate. Pyridoxamine phosphate, pyridoxine phosphate and deoxypyridoxine phosphate produced negligible reactivation of the enzyme. Deoxypyridoxine phosphate and, to a lesser extent, pyridoxine phosphate inhibited the reactivated enzyme, particularly when added to the inactivated enzyme before its reactivation with pyridoxal phosphate. Ethylenediaminetetraacetate (0.01[image]) did not inhibit serine synthesis by either normal or reactivated enzyme. Bi- and ter-valent cations did not stimulate serine synthesis; on the contrary, many cations were found to be strong inhibitors. Spectrophotometric evidence is presented for the formation of Schiff''s bases between pyridoxal phosphate and amino acids in neutral aqueous solution. These results are discussed in relationship to proposed mechanisms of enzymic reactions involving pyridoxal phosphate.