MANNITOL 1-PHOSPHATE MEDIATES AN INHIBITORY EFFECT OF MANNITOL ON THEACTIVITY AND THE TRANSLOCATION OF GLUCOKINASE IN ISOLATED RAT HEPATOCYTES

When tested in the presence of an inhibitor of sorbitol dehydrogenase, both mannitol and sorbitol caused a progressive inhibition of the det ritiation of [2-H-3]glucose in isolated rat hepatocytes. The purpose o f the present work was to investigate the possibility that this effect was mediated by the regulatory protein of glucokinase. When added to hepatocytes, mannitol decreased the apparent affinity of glucokinase f or glucose and increased the concentration of fructose required to sti mulate detritiation, without affecting the concentration of fructose 1 -phosphate. Its effect could be attributed to the formation of mannito l 1-phosphate, a potent agonist of the regulatory protein, which, simi larly to fructose 6-phosphate, reinforces its inhibitory action. Forma tion of mannitol 1-phosphate in hepatocytes was dependent on the prese nce of mannitol and was stimulated by compounds that increase the conc entration of glucose 6-phosphate. Liver extracts catalysed the convers ion of mannitol to mannitol 1-phosphate about 7 times more rapidly in the presence of glucose 6-phosphate than of ATP. The glucose 6-phospha te-dependent formation was entirely accounted for by a microsomal enzy me, glucose-6-phosphatase and was not due to a loss of latency of this enzyme. In hepatocytes in primary culture, mannitol decreased the det ritiation rate and counteracted the effect of fructose to stimulate gl ucokinase translocation. Taken together, these results strongly suppor t a central role played by the regulatory protein in the control of gl ucokinase activity and translocation in the liver, as well as a feedba ck control exerted by fructose 6-phosphate on this enzyme.