Inhibition of Papaya latex papain by photosensitive inhibitors. 1-(4,5-dimethoxy-2-nitrophenyl)-2-nitroethene and 1,1-dicyano-2-(4,5-dimethoxy-2-nitrophenyl)-ethene

1-(4,5-Dimethoxy-2-nitrophenyl)-2-nitroethene (1) was shown to be an irreve rsible inhibitor of papain (EC 3.4.22.2), causing a complete inhibition (12 0 min preincubation, pH 8.0), assuming that it attached to Cys-25 at the ac tive site of the enzyme (while a short preincubation time caused activation ). Only partial inhibition of papain was achieved, however, with 1,1 -dicya no-2-(4,5-dimethoxy-2-nitrophenyl)-ethene (2), a compound synthesized in th is work, which is also an irreversible inhibitor of papain. Since both comp ounds 1 and 2, and in each case of the inhibited enzyme, were 2-nitrobenzyl derivatives, they and the modified. enzyme were expected to be photosensit ive. Indeed, irradiation of the inhibited enzyme in the presence of mercapt oethanol resulted in a full recovery of the enzyme activity following inact ivation with compound 1 (similar to our previous finding with beta-galactos idase) and up to 67% recovery following inhibition with compound 2.