Possible Interactions of Sulfhydryl Groups and Quinonoid Compounds

If one considers the hypothetical possibilities for an interaction or a shielding effect of tocopherols and other quinonoid compounds on reactive sulfhydryl sites in enzyme molecules, two modes of action come to mind. In its capacity to act as an oxidation-reduction catalyst, tocopherol, in the form of an oxidized derivative, could interact with sulfhydryl groups and shift the equilibrium from the sensitive, reduced sulfhydryl toward the 

It is imaginable, in view of the significance of trace elements for the maintenance of tertiary structures and active sites of enzyme molecules, that the juncture between sulfhydryl groups and the active quinones is formed through coordination with a trace metal, for instance, Mn. In this currently, purely hypothetical case the various categories effective in our system, namely, sulfhydryl groups, trace elements, and quinones, would function jointly at a common site of action. 

The Enzymatic Block in Resi iicatoiiy Decline A. The a-KETOcLUTAHATE Dehydrogenase Complex 

From inhibitor studies at various intermediate steps of electron transfer and from other considerations, such as the substrate specificity of tocopherol-responsive respiratory decline, it seemed likely that the site of the main metabolic block leading to respiratory decline was not in the electron transfer chain proper, or in oxidative phosphorylation, as suspected earlier, but in certain dehydrogenase systems which connect the carboxylic acid cycle wdth the cytochrome chain. Using a-ketoglutarate as the substrate. 

It is possible that one or the other of those enzymes which we consider to be trace element sensitive sulfhydryl enzymes may actually contain selenium as the sensitive site. As mentioned above, selenium is one of the most potent bioelements known. In tissue it is found only in bound form. 

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VI. Possible Interactions of Sulfhydryl Groups and Quinonoid Compounds. 478... 

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