HEXOKINASE AND GLUCOKINASE BINDING IN PERMEABILIZED GUINEA-PIG HEPATOCYTES

The release of glucokinase (hexokinase IV) from digitonin-permeabilized hepatocytes from rat, guinea pig or mouse liver is inhibited by physiological concentrations of Mg2+ (> 0.25 mM). Preincubation of hepatocytes with fructose increases glucokinase release during permeabilization in the presence of Mg2+ but decreases glucokinase release in the absence of Mg2+, suggesting that fructose causes translocation of glucokinase from the Mg2+-dependent site. Glucose (25 mM) and sorbitol (1 mM) also induce translocation of glucokinase from the Mg2+-dependent site in guinea-pig, as in rat hepatocytes, but glucose is less effective than fructose or sorbitol, and the concentrations of fructose and sorbitol that cause half-maximal activation (A(50)) are 3-fold and 20-fold higher, respectively, in guinea-pig than in rat hepatocytes (170 mu M and 257 mu M, compared with 61 mu M and 13 mu M). Dihydroxyacetone and glycerol have no effect on fructose-induced or sorbitol-induced translocation in guinea-pig hepatocytes, in contrast with the potentiation and inhibition, respectively, by these substrates in rat hepatocytes. Some, but not all, of the differences between rat and guinea-pig hepatocytes could be due to the more reduced cytoplasmic NADH/NAD(+) redox state in guinea-pig cells. The activity of low-K-m hexokinases accounts for 30% of total hexokinase activity (low-K-m hexokinases + glucokinase) in guinea-pig hepatocytes. Of the low-K-m hexokinase activity, approx. 30% is released in the presence of Mg2+, 9 % shows Mg2+-dependent binding and 60 % shows Mg2+-independent binding. There was no substrate-induced translocation of low-K-m hexokinase activity, indicating that translocation is specific for hexokinase IV.