PHOSPHOGLUCOSE AND PHOSPHOMANNOSE ISOMERASE FLUXES IN THE LIVER-CELL

Phosphoglucose isomerase fluxes in the intact liver cell have been est imated from the extent of release of tritium on C-2 of glucose-6P, or the extent of incorporation of tritium onto C-2 of glucose-6P from tri tiated water. Phosphomannose isomerase fluxes have been estimated from the THO formation from [1T]galactose in excess of that formed in the glucose-6P dehydrogenase reaction, estimated using (14)CO2 production from [1-C-14]galactose. Liver phosphoglucose isomerase fluxes during g luconeogenesis from lactate in hepatocytes, or in fasted humans in viv o, range from about 10 to 14 times the net rate of glucose production. However, the reverse phosphoglucose isomerase flux, during gluconeoge nesis from 10 mM fructose in hepatocytes, is less than the net rate of gluconeogenesis. Reversible phosphomannose isomerase fluxes during gl uconeogenesis from lactate or dihydroxyacetone in vitro are somewhat l ower than net gluconeogenic fluxes, but this contribution to THO produ ction from sugars tritiated on C-1 can not be neglected.