ON THE MECHANISM OF THE AEROBIC OXIDATION OFD-XYLOSEAND OFD-RIBOSE-5-PHOSPHATE BY CELL-FREE EXTRACTS OF PSEUDOMONAS HYDROPHILA

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Abstract
Under aerobic conditions, cell-free extracts of Pseudomonas hydrophila, grown on D-xylose, oxidize D-xylose and D-ribose-5-phosphate by identical terminal pathways. Diphosphopyridine nucleotide-linked oxidation of either substrate is much more vigorous than the equivalent triphosphopyridine nucleotide-linked system. Sedoheptulose-7-phosphate is an intermediate in the presence of either substrate or cofactor. D-Glyceraldehyde-3-phosphate is oxidized in the presence of diphosphopyridine nucleotide with the concomitant uptake of the same amount of oxygen as with xylose or ribose-5-phosphate. Phosphoglycerate and phosphoenol pyruvate are rapidly converted to pyruvate, which, itself, disappears by a pathway not involving further oxygen uptake. Glyceraldehyde phosphate dehydrogenase is shown to be diphosphopyridine nucleotide specific and iodoacetate sensitive whereas glucose-6-phosphate and 6-phosphogluconate dehydrogenases are triphosphopyridine nucleotide specific and iodoacetate insensitive, both in fresh extracts and in ammonium sulphate fractions.The over-all oxidation is shown to proceed via D-glyceraldehyde-3-phosphate when diphosphopyridine nucleotide is used as coenzyme and most probably via the reactions of the pentose cycle in the presence of triphosphopyridine nucleotide. It is suggested that in the intact cell both processes occur simultaneously and compete with one another.