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Unlabeled prohistidine decarboxylase and prohistidine decarboxylase containing L-[carboxyl-(18)O]serine or L-[hydroxyl-(18)O]serine were isolated in homogeneous form from mutant 3 of Lactobacillus 30a grown with the appropriately labeled serine. There was no randomization or redistribution of label during growth, isolation of the protein, or enzymatic hydrolysis and reisolation of the labeled amino acids. These proteins were used to show that during proenzyme activation, in which individual pi subunits of the proenzyme are converted to alpha and beta subunits of the active enzyme [Formula: see text] (in which pi, alpha, and beta subunits have the partial structures shown and Prv designates a pyruvoyl group), no (18)O from H(2) (18)O is incorporated into the newly formed carboxyl terminus (Ser-81) of the beta chain, although no labilization of (18)O from proenzyme labeled with L-[carboxyl-(18)O]serine occurred when the proenzyme was activated in H(2) (16)O by the same procedures. The additional oxygen atom present in the carboxyl group of Ser-81 of the beta subunit is transferred from the hydroxyl group of Ser-82 of the proenzyme during the activation reaction. The same result was obtained with wild-type enzyme formed intracellularly. Peptide bond cleavage during activation of the proenzyme thus proceeds by a hitherto unobserved direct or indirect "serinolysis" coupled to alpha,beta-elimination at Ser-82 to yield the pyruvoyl group of the alpha subunit, rather than by hydrolysis. Possible mechanisms for the reaction are discussed briefly.

Conversion of prohistidine decarboxylase to histidine decarboxylase: Peptide chain cleavage by nonhydrolytic serinolysis