Presence of a Single Essential Histidyl Residue in Octopine Dehydrogenase as Shown by Photooxidation

Octopine dehydrogenase of Pecten maximus photooxidized in the presence of Rose Bengal was irreversibly inactivated following first‐order kinetics. The coenzyme and its analogues do not protect significantly against the photoinactivation whereas the substrates in the presence of coenzyme give an almost complete protection. About one –SH group was lost in the two cases. Moreover the native enzyme and the enzyme in which three –SH groups (including the essential one) were reversibly substituted were photoinactivated at the same rate. This suggests that the loss of the –SH group does not contribute to the photoinactivation of octopine dehydrogenase. About two histidines were destroyed in the photoinactivated enzyme and only one in the fully protected enzyme. It seems therefore that in octopine dehydrogenase only one histidine is essential and this residue can be protected in the ternary complexes. The partial protection effects of argi‐nine or pyruvate are additive when the two substrates are used together. This fact and the lack of protective effect of the coenzyme suggest that the essential histidine residue is near the substrates binding site.