Involvement of different cysteines in the inactivation of octopine dehydrogenase by p-chloromercuricphenyl sulfonic acid and o-phthalaldehyde.

Inactivation of octopine dehydrogenase by p-chloromercuricphenyl sulfonic acid (PCMS) and o-phthalaldehyde have been investigated. The activity loss due to the PCMS was faster than o-phthalaldehyde. PCMS associated inhibition was reversed by dithiothreitol completely which was not observed with o-phthalaldehyde inactivated enzyme. Fluorescence spectra of o-phthalaldehyde modified enzyme showed the formation of isoindole derivative with characteristic emission maximum at 410 nm. This derivative formation essentially involves cross-linking of proximal cysteine and lysine residues. Protection and selective reversible reaction studies have established that NADH prevents the enzyme against PCMS inactivation and the essential cysteine present at the NADH binding site is not involved in the o-phthalaldehyde reaction.