A reversible chemical modification of SH-groups of NADPH-cytochrome P-450 reductase is the subject of the present study. The enzyme was modified using first biradical RS-SR (R being the imidazolidine derivative) and a new affinity reductase inhibitor beta-cystamine adenosine diphosphate (ANSSN). These reagents were shown to be covalently bound to reductase SH-groups via the reaction of thiol-disulfide exchange resulting in the loss of reducing activity for cytochrome c. NADP+ protected reductase from inactivation and decreased the extent of the modification by RS-SR. The modification of reductase was reversible: the modified enzyme was partially reactivated with glutathione and dithiothreitol. The method proposed can be used to study both the reductase structure and the reversible inhibition of microsomal monooxygenase systems.
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