The Direct Oxidation Pathway in Plant Respiration.

Abstract

An important development in biochemistry has been the discovery of enzymes which can attack glucose-6-phosphate oxidatively to produce pentose phosphate and C02. The diversion of glucose-6phosphate by such a pathway has been variously called the hexose monophosphate shunt and the oxidative pathway and the names of several eminent research workers are linked with the elucidation of the reactions involved (reviewed by Horecker (9)). Recent work on a great variety of tissues, including higher plants (1, 2, 7, 8) has demonstrated that such systems are widespread. In view of this accumulating evidence, some workers have been led to consider that this metabolic byway may have importance in addition to its being a probable route to the pentoses in vivo, and the further elucidation of the reactions subsequent to pentose formation have lent color to this view. In particular, the recognition of the importance of sedoheptulose phosphate and its breakdown products has made possible the formulation of a cyclic sequence of events whose operation might of itself account for the total oxidation of glucose (9) and which would, at the least, produce metabolites which are also intermediates in the classical Embden-Meyerhof-Parnas (E.M.P.) glycolytic sequence. Although the details of the reactions remain to be clarified, the recent findings, to which experiments on higher plant tissues have contributed (1, 2, 7, 8), render the operation of the direct oxidation pathway in normal respiration quite plausible. There is ample evidence, of course, for the operation of the E.M.P. sequence of reactions in plant materials (11); at the present time, however, no evidence concerning the relative importance of the two alternatives in plant respiration has been offered. The present investigation was carried out to obtain evidence bearing on this point. Use was made of the fact that the method of glucose breakdown will determine the relative rates of release of its constituent carbon atoms. (For a discussion of the application of this principle see Bloom, Stetten, and Stetten (5) and Bloom and Stetten (6)). If the glucose molecule were broken down by glycolysis which includes equilibration at the tri?se level, two pyruvate molecules would be produced in which carbon atom 1 (C-l) and C-6 of the glucose would appear as the methyl carbons of the acid, C-2 and C-5 as the carbonyl carbons, and C-3 and C-4 as the carboxyl carbons. Carbons 1, 2, and 3 of the original glucose would, therefore, be indistinguishable respectively from 6, 5, and 4, and in the subsequent oxidative breakdown of the pyruvate, each member of a pair would appear in the respiratory C02 at the same rate as its partner. (Of the pairs, (C-3 and C-4) would be expected to appear first in the C02 followed by (C-2 and C-5) and then (C-l and C-6).) Thus, if comparable samples of tissue were respiring on glucose-1-C14 and glucose-6C14 respectively, the contribution of C14 to the C02 given off would be the same in each case. If, on the contrary, a glucose molecule was metabolized by way of the oxidative pathway, the C02 from the glucose1-C14 would be expected to be initially higher in C14 han that from the glucose-6-C14 since C-l of the glucose molecule is the first to be converted to C02. Provided that no assimilation of carbon residues containing different amounts of C-l and C-6 occurred, total yield of C1402 from the two glucose samples would be the same when oxidation was complete regardless of the path of breakdown. In the present experiments, it should be stressed, the time intervals were such that only a few percent of the supplied glucose was respired and the yields of C1402 are, therefore, regarded as initial as distinct from total. In these experiments, then, initial yields of C1402 from 2 glucose samples were compared, and the ratio, % radiochemical yield from glucose 6-C14/% radiochemical yield from glucose 1-C14 evaluated. Clearly, a ratio of near unity would indicate that schemes other than E.M.P. were playing a very minor part in glucose breakdown, whereas a ratio of less than unity would implicate the participation of the direct oxidation pathway.