EVIDENCE FOR CHANNELING OF INTERMEDIATES IN THE OXIDATIVE PENTOSE-PHOSPHATE PATHWAY BY SOYBEAN AND PEA NODULE EXTRACTS, YEAST EXTRACTS, ANDPURIFIED YEAST ENZYMES

Evidence is presented that intermediates of the oxidative pentose phos phate pathway (OPPP) are channeled from one pathway enzyme to the next . CO2 produced from [1-C-14]glucose in the presence of unlabelled path way intermediates contained much more radioactivity than predicted by a model in which pathway-produced intermediates are in equilibrium wit h identical molecules in the bulk phase. This was the case whether glu cose 6-phosphate (Glc6P), 6-phosphogluconolactone, or 6-phosphoglucona te was added. Assumptions involved in calculating the amount of (CO2)- C-14 predicted for free mixing of C-14-labelled and unlabelled interme diates are discussed, together with the following results. (a) (CO2)-C -14 production by pea nodules in the presence of 3 mM 6-phosphoglucona te was higher than in its absence. (b) Apparent chaneling of intermedi ates was much higher for purified yeast enzymes than for yeast extract . (c) 6-Phosphogluconate and 6-phosphogluconolactone were channeled be tween yeast Glc6P dehydrogenase and 6-phosphogluconate dehydrogenase d espite the absence of 6-phosphogluconolactonase in the purified yeast enzyme mixture. (d) When purified yeast hexokinase was physically sepa rated from Glc6P dehydrogenase and 6-phosphogluconate dehydrogenase by a dialysis membrane, there was no apparent channeling. (e) Poly(ethyl ene glycol), high salt and detergents had little effect on apparent ch anneling of OPPP intermediates, which is consistent with a stable comp lex of enzymes. On the other hand: density gradient centrifugation exp eriments suggested a more transient interaction between the enzymes. T aken together, the results support channeling of OPPP pathway intermed iates.