Acceptors

ACCEPTORS (from Latin accipere to accept), substances which, participating in oxidation-reduction reactions proceeding at the expense of the elements of water, "take upon themselves" these elements, adding either OH or H of water to themselves. As an example of a coupled oxidation-reduction reaction, one can point to the case of the oxidation of hypophosphorous acid in the presence of metallic palladium. Salts of the indicated acid do not decompose water by themselves at a measurable rate, just as metallic palladium does not decompose it either. But if a negligible amount of palladium black is introduced into an aqueous solution of these salts, hypophosphorous acid will immediately begin to be oxidized to phosphorous acid, and at the same time hydrogen will be liberated on the palladium: H₃PO₂ + H₂O = H₃PO₃ + H₂. In this case, hypophosphorous acid adds to itself (accepts) the OH of water, while palladium "takes upon itself" the H. The oxidation of formaldehyde in the presence of methylene blue proceeds completely analogously. The aldehyde is oxidized by the OH of water into formic acid, while the H of water reduces methylene blue to its leuko base. In this case, methylene blue is a hydrogen acceptor. Hydrogen acceptors play a very essential role in the respiration process. The oxidation of various organic substances serving as material for respiration, taking place in the living cell, in the overwhelming majority of cases cannot be carried out with the aid of gaseous oxygen, but occurs exclusively at the expense of the OH of water. However, the indicated process will take place only in the case when the H formed in this process is removed with the aid of one or another acceptor. In the plant cell, such hydrogen acceptors are the respiratory pigments that are well studied at the present time. In the animal cell, among the substances that can serve as hydrogen acceptors, so far only compounds containing the disulfide group -S-S- (e.g., cystine) have been studied, which pass upon the addition of H into the sulfhydryl group -SH (e.g., in cysteine): -S-S- + H₂ = -SH + HS-. To the binding of H by the corresponding acceptor, there is joined, as a further phase, the removal of H from the reduced acceptor by molecular oxygen (apparently, after preliminary activation of the latter). By this, the process of aerobic, oxybiotic, oxygen respiration is carried out, in which, consequently, molecular oxygen serves as the final hydrogen acceptor.

Oxidation processes play a dominating role in the cell economy; however, alongside them, various reduction processes also proceed. The hydrogen required for them is also taken at the expense of the elements of water under the condition that the hydroxyl will be simultaneously bound by some oxygen acceptor.