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1. The self-catalysed hydrolyses of the thiol esters, S-hippurylthioglycollic acid and S-ethyl monothiolsuccinate, have been shown to be slower than the deacylation step for the papain-catalysed hydrolysis of hippuric esters, by a factor approx. 10(5). This difference in rate constants largely reflects a difference in activation energy, which together with other evidence drawn from the literature make it unlikely that a carboxylate ion could be the nucleophile responsible for the deacylation of acyl-papain. 2. The imidazole-catalysed hydrolysis of S-hippurylthioglycollic acid and ethyl thiolacetate have activation energies similar to that for the deacylation step in papain-catalysed hydrolyses. This, together with other evidence drawn from the literature, suggests that the imidazole of a histidine residue is the nucleophile responsible for the deacylation of acyl-papain.

A STUDY OF SOME THIOL ESTER HYDROLYSES AS MODELS FOR THE DEACYLATION STEP OF PAPAIN-CATALYSED HYDROLYSES