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

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Abstract
Unlabeled prohistidine decarboxylase and prohistidine decarboxylase containing L-[carboxyl-18]serine or L-[hydroxyl-18O]serine were isolated in homogeneous form from mutant 3 of Lactobacillus strain 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, no 18O from H218O is incorporated into the newly formed carboxyl terminus (Ser-81) of the .beta. chain, although no labilization of 18O from proenzyme labeled with L-[carboxyl-18O] serine occurred when the proenzyme was activated in H216O 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.