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Abstract

The conversion of oxidation energy into the energy-rich phosphoryl linkage of ATP has been demonstrated in a system consisting of the tertiary-butylammonium salts of ADP and Pi in pyridine and utilising several mercapto carboxylic acids or their disulfides as mediators and iodine or hemin or hemindimethyl ester as oxidants. Iodine with several thiols in 1:1 molar proportions produces relatively poor yields of ATP (1.5–5.4%) which can be considerably increased (10–15%) when the corresponding disulfides are used, whereas a thioether (N-acetylmethionine) had no effect. This led to the conclusion that disulfides are better substrates and accordingly much higher amounts of ATP were formed from all thiols on using a two-fold amount of oxidant (up to 25.6%). Taking this into account a mechanism has been formulated in which the formation of a phosphorylating species, RS-OPO3H2, is assumed to be formed from disulfides. Hemin and (less efficiently) its dimethyl ester used as oxidants also give rise to the formation of ATP in our system, but only under aerobic conditions. Here optimal yields (up to 16.7%) were obtained with thioglycolate or its disulfide as mediators, a thioether (N-acetylmethionine) being almost ineffective, and no difference in efficiency between thiol acid and disulfide was observed at a hemin: sulfur ratio of 1:1. The yields depend in some way on the hemin: S ratio, lower values giving higher amounts of ATP on oxidation of the mercapto compound, but not of its disulfide. Therefore a reaction mechanism different from that of iodine must be presumed. The formation of the same phosphorylating species as above has been assumed to occur preferently with a thiol and to a lesser extent with a disulfide. The role of oxygen is still rather obscure, for it can not be determined whether it is the terminal electron acceptor or electron carrier. A catalytic participation of hemin could not be demonstrated, perhaps on account of the slow autoxidation rate of the ferrous heme iron.