METHYLAMINE DEHYDROGENASE IS A LIGHT-DEPENDENT OXIDASE

Quinoproteins may function either as oxidases or dehydrogenases, depen ding on the nature of their quinone prosthetic group. The tryptophan t ryptophylquinone (TTQ)-linked methylamine dehydrogenase (MADH) is rela tively inert towards O-2 in its reduced form. It is shown that on expo sure to long range UV (ultaviolet) light, MADH is oxidized in the pres ence of O-2 and exhibits substrate-dependent steady-state oxidase acti vity. The effects of light are completely reversible, and oxidase acti vity is lost when the light is turned off. The Light-dependent oxidati on of MADH proceeds via a semiquinone intermediate which accumulates t o near stoichiometric levels. The absorption of the light appears to p rovide energy to overcome a thermodynamic barrier to the transfer of a n electron from TTQ to O-2. These studies provide a basis for understa nding what factors dictate whether an oxidoreductase is a dehydrogenas e or an oxidase. (C) 1998 Elsevier Science B.V. All rights reserved.