Isotopic Hydrogen Exchange in Reactions Catalysed by Cysteine Lyase and Serine Sulphhydrase

Abstract

Serine sulphhydrase from chicken liver and cysteine lyase from chicken‐embryo yolk sae catalyse the exchange of α‐H atoms of the amino acid substrate with 3H2O. The degree of labelling of the unreacted substrate approaches a maximum of one atom per mol of amino acid. In the absence of replacing agent there is practically no H‐exchange in the substrate. The α‐H of the accumulating β‐substitution product is completely replaced by the labelled hydrogen of the solvent water, irrespective of the duration of incubation. The amount of labelled α‐hydrogen incorporated into excess (unreacted) amino acids substrate within 3.5‐h incubation is somewhat less than the amount incorporated into the product of the complete enzymic β‐replacement reaction. Within the sensitivity limits of detection, the enzymes do not induce any isotopic exchange either of β‐H atoms in the amino substrate or of 18O‐labelled β‐HO groups, in the case of l‐serine. Neither serine sulphhydrase nor cysteine lyase will catalyse α‐hydrogen exchange in close structural analogues of their substrates, e.g. l‐alanine, d‐serine, threonine, 3‐phosphoserine. A special case is the interaction of cysteine lyase with the competitive inhibitor, l‐serine (whose inhibitor constant, Ki, is equal to the Michaelis constant, Km, of l‐cysteine): the lyase catalyses, only in presence of a cosubstrate thiol, α‐H exchange in l‐serine at approximately the same rate as in l‐cysteine. The present data concerning isotopic α‐H exchange in substrate amino acids, and evidence published earlier, suggest that the catalytic mechanism of replacement‐specific β‐lyases may significantly differ from that of the eliminating or ambivalent (mixed‐function) lyases. Formation of α,β‐unsaturated pyridoxylidene aldimines as real reaction intermediates is unlikely in the case of lyases specifically catalysing β‐replacement reactions; these may proceed by some alternative mechanism of the type suggested in this paper. Copyright © 1975, Wiley Blackwell. All rights reserved