ON THE MECHANISM OF GLUCOSE-6-PHOSPHATE OXIDATION IN CELL-FREE EXTRACTS OF XANTHOMONAS PHASEOLI (XP8)

Abstract

Extracts of the plant pathogenic organism Xanthomonas phaseoli contain both TPN- and DPN-linked glucose-6-phosphate dehydrogenases as well as a DPN-specific glyceraldehyde phosphate dehydrogenase. DPNH oxidase is very active and sensitive to high concentrations of cyanide. No evidence could be found for the presence of pyridine nucleotide transhydrogenase.Ribose-5-phosphate is metabolized via the pentose cycle; sedoheptulose phosphate is an intermediate and other criteria of the functioning of the pentose cycle are met when ribose-5-phosphate is the substrate. The same system also functions with 6-phosphogluconate as substrate but at a level which suggests that this pathway is not of great significance.The Entner–Doudoroff 6-phosphogluconate-splitting pathway is the major avenue of hexosephosphate utilization in cell-free extracts. Pyruvate formation and utilization are discussed in relation to function and cofactor requirements.While phosphoglucose isomerase, aldolase, triosephosphate isomerase, and the enzymes converting phosphoglyceraldehyde to pyruvate are present and enzymatically active, phosphofructokinase appears to be the "bottleneck" to glycolysis. On the other hand fructose-1,6-diphosphate is oxidized by both DPN- and TPN-linked pathways, the latter through the efficient functioning of a specific phosphatase which forms fructose-6-phosphate readily. Glycolysis is considered to play only a very minor role in these extracts.A new "hexose cycle" for the metabolism of hexosephosphate is proposed. It consists of oxidation through "Zwischenferment", followed by the Entner–Doudoroff splitting system and resynthesis of hexosemonophosphate through reversal of the aldolase reaction followed by specific carbon-1 phosphate hydrolysis of fructose-1,6-diphosphate. The cycle is shown to be metabolically feasible and thermodynamically likely. It is considered to be the pathway by which cells which tend not to utilize the pentose cycle or classical glycolysis, but have an active 6-phosphogluconate-splitting system, metabolize hexosephosphate. Pyruvate produced as a result of the operation of this system is then oxidized via the tricarboxylic acid cycle in glucose-grown cells.