The inhibition of mitochondrial complex I (NADH:ubiquinone oxidoreductase) by Zn2+

NADH: ubiquinone oxidoreductase (complex I) from bovine heart mitochondria is a highly complicated, membrane-bound enzyme. It is central to energy transduction, an important source of cellular reactive oxygen species, and its dysfunction is implicated in neurodegenerative and muscular diseases and in aging. Here, we describe the effects of Zn2+ on complex I to define whether complex I may contribute to mediating the pathological effects of zinc in states such as ischemia and to determine how Zn2+ can be used to probe the mechanism of complex I. Zn2+ inhibits complex I more strongly than Mg2+, Ca2+, Ba2+, and Mn2+ to Cu2+ or Cd2+. It does not inhibit NADH oxidation or intramolecular electron transfer, so it probably inhibits either proton transfer to bound quinone or proton translocation. Thus, zinc represents a new class of complex I inhibitor clearly distinct from the many ubiquinone site inhibitors. No evidence for increased superoxide production by zinc-inhibited complex I was detected. Zinc binding to complex I is mechanistically complicated. During catalysis, zinc binds slowly and progressively, but it binds rapidly and tightly to the resting state(s) of the enzyme. Reactivation of the inhibited enzyme upon the addition of EDTA is slow, and inhibition is only partially reversible. The IC50 value for the Zn2+ inhibition of complex I is high (10-50 mu M, depending on the enzyme state); therefore, complex I is unlikely to be a major site for zinc inhibition of the electron transport chain. However, the slow response of complex I to a change in Zn2+ concentration may enhance any physiological consequences.