Quinone interaction with the respiratory chain-linked NADH dehydrogenase of beef heart mitochondria. II. Duroquinone reductase activity

1. Enzymatic reduction of 2,3,5,6-tetramethyl-1,4-benzoquinone (duroquinone) by NADH can be used in an assay procedure for the NADH dehydrogenase. The reduction of this quinone occurs in the region of the electron transport system between the primary dehydrogenase and the cytochrome system as defined by the almost complete loss of reductase activity following piericidin A treatment.

2. Duroquinone reduction can be distinguished from ubiquinone 2 reduction by the marked inhibition of the former following phospholipase C, poly- -lysine, or chloroquine diphosphate treatment. In addition, duroquinone reduction requires the presence of endogenous ubiquinone 10 specifically whereas ubiquinone 2 reduction does not require the presence of endogenous quinone. These observations are consistent with the nonequivalency of the reduction sites of duroquinone and ubiquinone 2.

3. Duroquinol can be utilized as an electron donor for the energy-linked reduction, of NAD⁺. Duroquinol reduction of NAD⁺ is dependent upon the presence of ATP, is inhibited by oligomycin, carbonyl cyanide p-trifluoro methoxyphenylhydrazone and piericidin A, and is not inhibited by antimycin A at levels which inhibit electron transport.

4. Duroquinone reduction as well as ubiquinone 2 reduction are inhibited almost completely by phospholipase A, p-chloromercuribenzoate, o-phenanthroline, and Triton X100 treatments.