On the mechanism of enzyme inhibition by sulfhydryl reagents.

In recent years a great deal of evidence has accumulated to show that intact -SH groups are necessary for the full activity of a variety of enzymes. The exact function of the thiol group in the protein part of these “sulfhydryl enzymes,” however, remains obscure. It has been suggested that the -SH group directly participates in the activity of these enzymes, and that it might be actually located at the “active center” where combination of the enzyme and substrate takes place (1). This view is supported mainly by experiments on the protection of enzymes from the action of -SH inhibitors (reagents capable of selectively inhibiting -SH enzymes) by their substrate, prosthetic group, or analogue of the substrate (2). If the sole function of the sulfhydryl group in these enzymes was to provide a locus for the combination of the substrate with the enzyme protein, then the inhibition of a sulfhydryl enzyme by a specific SH reagent should be the same, regardless of the substrate used for assay, provided that the enzyme is the limiting factor in the activity determination. To the author’s knowledge there are no data in the literature on this question. This paper is an account of some experiments on wheat germ lipase and n-amino acid oxidase designed to test this point.