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

β-Galactosidase (EC 3.2.1.23) is known to be inhibited by some thiol reagents. 1-Benzoyl-1-cyano-2-(4,5-dimethoxy-2-nitrophenyl)-ethene (1) was shown to be an irreversible inhibitor, whine 1, 1-dicyano-2-(4,5-dimethoxy-2- nitrophenyl)-ethene (2) was demonstrated as a positive irreversible modulator causing a rise of up to 186% in β-galactosidase activity. Compound 2 is, however, an irreversible inhibitor of the cysteine proteinase papain (preceding paper). Kinetic values of β-galactosidase at pH 8.3 with o- nitrophenyl β-D-galactopyranoside (ONPG) as the substrate and for compounds 1 and 2 were determined and in view of model experiments, it was assumed that both compounds possibly reacted with the thiol side chain of Cys in the active site inducing allosteric changes in the enzyme. Since the enzyme, modified by compound 1 or 2, was a 2-nitrobenzyl derivative, near-UV irradiation resulted in a recovery of up to 91% and a reduction of the enzyme's activity to 90%, respectively.