The Possible Function of Quinols, Chromanols and Chromenols in Intracellular Respiration

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Abstract
A survey is given of the chemical properties of four biochemically important groups of compounds: the vitamins E, the vitamins K, the ubiquinones and plastiquinones. All four groups are known in the form of the p-benzoquinone. The isomeric chromenol has been described in the ubiquinone and plastoquinone series. All four p-quinones can be reduced to the quinol, or, an acid solution, to the chromanol (the chromanol of plastoquinone has not yet been described). Only a few of these compounds are known with certainty to be present in nature—the quinone in the vitamin K, ubiquinone and plastoquinone series, the quinol in the ubiquinone and the chromanol in the vitamin E series. The possible biochemical function of α-tocopherol and ubiquinone, both of which are localized in the mitochondnia, is discussed. There is no direct evidence that α-tocopherol has any function in intracellular respiration, but the possibility remains open. There is good reason to believe that ubiquinone is a component of the respiratory chain operating between the flavoproteins and the cytochrome system. The possible function of vitamin K in intracellular respiration is also discussed. Possible reaction schemes for the participation of quinones and related compounds in oxidative phosphorylation are given.

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