Inhibition of glucokinase by alloxan through interaction with SH groups in the sugar-binding site of the enzyme.

Alloxan is known to inhibit pancreatic B cell and liver glucokinase and glucose protects the enzyme against inhibition. The dithiol 1,4-dithiothreitol (1,4-DTT) protected against and reversed the inhibition of glucokinase by alloxan. An investigation into the structure-activity relationship using a variety of different dithiols demonstrated that the ability of the dithiols to protect against and to reverse the inhibition of glucokinase by alloxan was dependent on the spacing between the SH (thiol) groups of the various dithiols. Only 1,3-dimercaptopropane, 1,4-dimercaptobutane, 1,4-dithioerythritol, and 1,4-DTT, with intermediate spacing between the SH groups, reversed the inhibition of glucokinase induced by alloxan. Dithiols with two vicinal SH groups such as 1,2-dimercaptoethane and 2,3-dimercaptopropanol (BAL) were ineffective in the same way as dithiols with more widely spaced SH groups such as 1,5-dimercaptopentane and 1,6-dimercaptohexane. Except for 1,6-dimercaptohexane, all dithiols also protected glucokinase against the inhibition of alloxan. The monothiol cysteine, but not glutathione, a tripeptide monothiol, also protected glucokinase against alloxan inhibition but both were unable to reverse the inhibition. Like alloxan, other dithiol reagents such as ninhydrin, N-ethylmaleimide, and maleimide inhibited glucokinase. Glucose and 1,4-DTT protected glucokinase against this inhibition. 1,4-DTT partially reversed this inhibition. It is concluded, therefore, that the mechanism of inhibition of glucokinase by alloxan is a reaction of alloxan with two adjacent SH groups in the depth of the sugar-binding site of the glucokinase, with formation of a disulfide bond and concomitant inactivation of the enzyme. Because glucokinase can couple changes in the blood glucose concentration to changes in the glycolytic flux rate and corresponding changes in the rate of insulin secretion, it may function as a glucose signal recognition enzyme in the pancreatic B cell. This mechanism of interaction of alloxan with glucokinase may thereby provide an explanation for the ability of alloxan to inhibit glucose-induced insulin secretion.