COENZYME-Q REDUCTASE FROM LIVER PLASMA-MEMBRANE - PURIFICATION AND ROLE IN TRANS-PLASMA-MEMBRANE ELECTRON-TRANSPORT

A specific requirement for coenzyme Q in the maintenance of trans-plas ma membrane redox activity is demonstrated. Extraction of coenzyme Q f rom membranes resulted in inhibition of NADH-ascorbate free radical re ductase (trans electron transport), and addition of coenzyme Q(10) res tored the activity, NADH-cytochrome c oxidoreductase (cis electron tra nsport) did not respond to the coenzyme Q status. Quinone analogs inhi bited trans-plasma-membrane redox activity, and the inhibition was rev ersed by coenzyme Q. A 34-kDa coenzyme Q reductase (p34) has been puri fied from Pig-liver plasma membranes. The isolated enzyme was sensitiv e to quinone-site inhibitors. p34 catalyzed the NADH-dependent reducti on of coenzyme Q(10) after reconstitution in phospholipid liposomes. W hen plasma membranes were supplemented with extra p34, NADH-ascorbate free radical reductase was activated but NADH-cytochrome c oxidoreduct ase was not. These results support the involvement of p34 as a source of electrons for the trans-plasma-membrane redox system oxidizing NADH and support coenzyme Q as an intermediate electron carrier between NA DH and the external acceptor ascorbate free radical.