Photoreversible modulators of Escherichia coli beta-galactosidase. 1-benzoyl-1-cyano-2-(4,5-dimethoxy-2-nitrophenyl)-ethene and 1,1-dicyano-2-(4,5-dimethoxy-2-nitrophenyl)-ethene

beta-Galactosidase (EC 3.2.1.23) is known to be inhibited by some thiol rea gents. 1-Benzoyl-1-cyano-2-(4,5-dimethoxy-2-nitrophenyl)-ethene (1) was sho wn to be an irreversible inhibitor, while 1, 1-dicyano-2-(4,5-dimethoxy-2-n itrophenyl)-ethene (2) was demonstrated as a positive irreversible modulato r causing a rise of up to 186% in beta-galactosidase activity. Compound 2 i s, how ever, an irreversible inhibitor of the cysteine proteinase papain (p receding paper). Kinetic values of beta-galactosidase at pH 8.3 with o-nitr ophenyl beta-D-galactopyranoside (ONPG) as the substrate and for compounds 1 and 2 were determined and in view of model experiments, it was assumed th at both compounds possibly reacted with the thiol side chain of Cys in the active site inducing allosteric changes in the enzyme. Since the enzyme, mo dified by compound 1 or 2, was a 2-nitrobenzyl derivative, near-UV irradiat ion resulted in a recovery of up to 91% and a reduction of the enzyme's act ivity to 90%, respectively.