Abstract
Coenzyme Q10 and menadione have been shown to enhance the succinic dehydrogenase activity of rat liver homogenates and frozen sections as measured quantitatively by the reduction of INT. Increased formazan deposition as estimated by visual examination was also found in sections of liver and other tissues in comparable experiments with the two quinones in which the tetrazolium salt Nitro-BT was employed.
Extraction of sections with acetone decreased the succinic dehydrogenase activity of liver sections by approximately two-thirds of that present in nonextracted sections as measured by INT reduction. The addition of coenzyme Q10 or menadione restored the activity to essentially that present in nonextracted sections incubated in the presence of the same constituents. These data suggest that a quinone, presumably coenzyme Q10, acts as an intermediate electron transport agent between succinic dehydrogenase and INT or Nitro-BT.
The effects of coenzyme Q10 and menadione on the activities of diphosphopyridine nucleotide (DPNH) diaphorase, triphosphopyridine nucleotide (TPNH) diaphorase, α-glycerophosphate dehydrogenase and monoamine oxidase, all of which are demonstrable by tetrazolium salt reduction procedures, have been studied. Two of these enzymes, TPNH diaphorase and monoamine oxidase, show no significant enhancement of activity in the presence of coenzyme Q10, and a relatively slight enhancement in the presence of menadione. α-Glycerophosphate dehydrogenase shows a pattern of enhancement comparable to that of succinic dehydrogenase. DPNH diaphorase activity, while enhanced by both of the quinones, shows a lesser degree than is found with the succinic dehydrogenase and α-glycerophosphate dehydrogenase systems.
The diverse effects of coenzyme Q10 and menadione on the five enzyme systems investigated in the present work indicate that the two quinones do not simply act as nonspecific intermediate electron carriers between dehydrogenase system and tetrazolium salt, but that they enhance enzyme activity by a more specific interaction with some component of the enzyme system.