ON THE MECHANISM OF GLUCOSE METABOLISM IN THE PLANT TUMOR-INDUCING ORGANISM AGROBACTERIUM TUMEFACIENS

Abstract

Extracts of the plant tumor-inducing organism Agrobacterium tumefaciens do not oxidize glucose directly to gluconic or to 2-ketogluconic acid. Such extracts contain an extremely active, cyanide-sensitive DPNH oxidase. TPNH oxidase shows little activity and pyridine nucleotide transhydrogenase appears to be absent. Hexokinase is active, glucose-6-phosphate dehydrogenase functions either as a DPN- or as a TPN-linked system, 6-phosphogluconic acid dehydrogenase is TPN-specific, and glyceraldehyde-3-phosphate dehydrogenase is DPN-specific.Evidence is presented to show that the pentose cycle operates in such preparations and also that the extent to which it functions is determined by the rate-limiting dehydrogenation and decarboxylation of 6-phosphogluconate. Thus, heptulose phosphate is formed readily from ribose-5-phosphate and much less effectively from 6-phosphogluconate. The net formation of hexose as a product of the cycle is also shown. It was possible to demonstrate under certain conditions that an effective competition exists between glyceraldehyde-3-phosphate dehydrogenase and transaldolase for available triose phosphate.From reaction rates and other data it is clear that the Entner–Doudoroff 6-phosphogluconate-splitting pathway is the major avenue of hexose phosphate utilization in cell-free extracts. Pyruvate thus formed is oxidized via the tricarboxylic acid cycle while glyceraldehyde-3-phosphate can be changed slowly into pyruvate or be recycled into hexose phosphate. The latter pathway is facilitated by the existence of a highly active fructose-1,6-diphosphatase.Whereas phosphohexoseisomerase is found to be highly active, phosphofructokinase shows very little activity, aldolase does not appear to be very effective, and the conversion of phosphoglyceric acid to pyruvate is extremely slow. For these reasons, glycolysis is considered to play a very minor role in glucose metabolism in these extracts.Data on the effects of some carbohydrate inhibitors on glucose-6-phosphate dehydrogenase and on phosphohexoseisomerase are also given.