Intermediates of the Pentose Phosphate Pathway as Respiratory Substrates.

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Abstract
It has been established that D-xylose and D-ribose can be utilized as respiratory substrates by a variety of plant tissues. One of the most active was the corn cole-optile, and this material was used in further experiments which showed that sedoheptulose and gluconate are also respired. Each gives rise to alcohol and CO2 under nitrogen, although the utilization of gluconate, in contrast to that of the pentoses and sedoheptulose, is markedly reduced under these conditions. C14 appeared both in the alcohol and in the CO2 produced anaerobically (and aerobically in the presence of arsenite) from ribose-l-Cl4 and xylose-1-C14. In addition, it was observed that the tracer was characteristically distributed in these products; roughly 2/3 appeared in the CH3 of the alcohol and 1/3 in the CO2. These findings are interpreted to mean that the pentoses were being metabolized by way of the pentose phosphate cycle. Experiments with gluconates labeled either in the 1-, 2- or 6- positions showed that release of C-1 is an early reaction in its utilization. The pattern of labeling in the alcohol and CO2 produced anaerobically from the three gluconates is also consistent with its conversion to pentose by loss of C-l in an oxidative step and subsequent conversion to glucose in the pentose phosphate cycle. As evidence for the presence of such a cycle in the tissue a more incisive finding is that the paths taken by individual C-atoms of the labeled substrates, as indicated by the distribution in the products of anaerobic respiration, are essentially those predicted from the present knowledge of the reaction sequence.