Reactions of Coenzyme Q in the DPNH Dehydrogenase Segment of the Respiratory Chain

Despite extensive studies in many laboratories, neither the exact role of CoQ in electron transport nor its site of action are unambiguously established.Although it is generally agreed that CoQ is a functional component of the respiratory chain,1' 2 pending the resolution of the questionswhether or not the oxido-reduction of internal CoQjo is sufficiently rapid to be on the main path of electron flow from flavo- protein to 02, alternative functions,5 6 for example, electron transport between functionally interconnected respiratory chains,7 cannot be precluded.At present there appears to be general agreement6' 8 that the reduction of both endogenous and external CoQjo by DPNH is inhibited by Amytal and their reoxidation is inhibited by antimycin A. This localizes the main reaction site of the natural homologue between flavoprotein and cytochrome c,, but neither the identity of the immediate reductant nor that of the oxidant is firmly established.According to a recent re- port9 short-chain CoQ homologues can also react in the cytochrome c region of the chain.In one scheme' for the DPNH oxidase chain, CoQ is shown to accept electrons from DPNH dehydrogenase by way of nonheme iron.Since all the nonheme iron in this segment of the chain belongs to DPNH dehydrogenase'' 11 and is functional in that enzyme,'2 it would follow that the dehydrogenase is the immediate reaction partner of CoQ.Evidence favoring this view is that a cytochrome-free particle, the DPNH-CoQ reductase complex"3 ("complex 1") catalyzes the Amytal-sensitive reduction of short-chain CoQ homologues, although not of external CoQ10.This preparation, however, also contains components other than the flavoprotein (cf.below).Further, Pharo and Sanadi'4-'6 have described a soluble preparation ("DPNH-ubiquinone reductase"), extracted from ETPH by the acid-ethanol-heat procedure previously used'7-'9 for the isolation of DPNH-cytochrome reductase from closely related particles.While ubiquinone reductase has been reported to catalyze the Amytal'5and rotenone'6-sensitive reduction of CoQ homologues by DPNH, DPNH-cytochrome reductase is thought'6 to be devoid of such activity.This divergent behavior is puzzling in view of the great similarities in the extraction and isolation procedures and in starting materials.Retention of the Amytal-and rotenone-sensitive CoQ reduction through this extraction procedure is also paradoxical, since it has been shown20 that the same method, whether applied to particles or to the purified DPNH dehydrogenase, fragments the flavoprotein, with attendant changes in the molecular and catalytic properties, including loss of its characteristic ferricyanide activity, substrate-inducible EPR signal'2 at g = 1.94, extensive loss of nonheme iron and of "labile" sulfide,2' and the emergence of cytochrome c and DCIP reductase activities.The low molecular weight and low nonheme iron and "labile" sulfide content of ubiquinone reductase'4-'6 and the absence of the g = 1.94 signal22 indicate, in fact, extensive degradation.The observation22 that the reduction of CoQ homologues by the purified dehydrogenase is very slow and is 467