Photo-induced covalent labelling of malate dehydrogenase by quercetin

Quercetin powerfully inhibits malate dehydrogenase reversibly and cooperatively with 50% inhibition at 2.5μM at pH 7.50. Irradiation with light of wavelengths ≳350nm, of a mixture of malate dehydrogenase and quercetin leads to covalent inhibition whose extent is directly related to quercetin concentration, inversely related to enzyme concentration, and partially protected against by NADH. Prephotolysis of quercetin followed by incubation (in the dark) with malate dehydrogenase led to a time-dependent covalent inhibition.