REACTIONS OF COENZYME Q IN THE DPNH DEHYDROGENASE SEGMENT OF THE RESPIRATORY CHAIN

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Abstract
The properties of DPNH dehydrogenase and of the DPNH- CoQ reductase complex were compared. The only difference in catalytic activity detected was the ability of the latter to catalyze the rotenone- and Amytal-sensitive reduction of external, short-chain CoQ homologues. As judged by the full reactivation of this reaction by mitochondrial lipids, following inactivation by phospholipase A, the reduction of CoQ by DPNH requires lipids which are not present in the purified flavoprotein. DPNH-cytochrome c and DPNH-ubiquinone reductases from heart mitochondria were compared, and no major difference in properties was detected. The reactivity of DPNH dehydrogenase with CoQ1, like its cytochrome reductase activity, is very low. On thermal or acid-ethanol modification of the enzyme, however, a large increase in CoQ1 reductase activity occurs. The acid-ethanol fragmentation product also catalyzes a partially Amytal- and rotenone-sensitive reduction of CoQ6.