14C labelling of octulose bisphosphates by L-type pentose pathway reactions in liver in situ and in vitro.

The complete reaction sequence of the pentose pathway in vitro was studied by incubating [1-14C] ribose 5-phosphate with rat liver enzyme preparation and assessed by both the rate and extent of formation of the glucose 6-P product. The reactions formed, as intermediates, the 1,8-bisphosphates of D-glycero D-ido octulose (D-g D-i Oct) and D-glycero D-altro octulose, both heavily labelled at C-4 with 14C isotope during the 12h incubation. The formation of the octulose phosphates and the specificity of their isotopic labelling confirms an important prediction of, and contribution by reactions of the L-type pentose phosphate pathway (L-PP) in liver in vitro. Infusion in situ of [6-14C] glucose into the liver of the anaesthetized rabbit resulted in the formation of high specific activity [8-14C] D-g D-i Oct 1,8-P2. The specificity of labelling indicates that the octulose intermediate is formed according to the options of the L-PP mechanism of glucose metabolism in intact liver.