Erythro peptidyl epoxides are selective inactivators of cysteine proteases. The alkylation site, both on the enzyme papain and on the epoxide itself, was characterized. The inactivation of papain with the peptidyl epoxide erythro benzyloxycarbonyl-Phe-Ala-epoxide was followed by total hydrolysis by acid. Mass spectral analysis of the hydrolysate revealed, in addition to the expected amino acids, a unique signal of m/z 209 (MH+). Its high-resolution mass spectrum and daughter peak analysis correspond to the product of alkylation on cysteine and the expected fragmentation. A similar MS pattern was obtained for a synthetic model compound corresponding to the expected hydrolysis product. A 13C NMR analysis of papain inactivated by a specifically 13C-labelled peptidyl epoxide indicated that the alkylation of the enzyme's cysteine residue occurs on the primary carbon of the epoxide moiety.

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