Mitochondrial membrane-bound hexokinase of ascites tumor cells. Functional implications of lysine residues studied by modification with imidoesters.

Conformational changes of hexokinase from ascites tumor cells have been studied by chemical modification of lysine residues with imidoesters with the following results: 1) The membrane-bound enzyme, in contrast to the soluble enzyme, is not inactivated by treatment with dimethyl suberimidate, which suggests (a) lysine residue(s) essential for the activity that is protected in the membrane-bound enzyme. 2) Three different conformations have been detected in the membrane-bound enzyme. Two of these are induced by glucose and glucose 6-phosphate, respectively. 3) Treatment of the membrane-bound enzyme with dimethyl suberimidate affects its sensitivity to the inhibition by glucose 6-phosphate, but not its activity or degree of maximal inhibition. This suggests that lysine(s) is related to the binding of glucose 6-phosphate to its allosteric regulatory site. 4) In intact tumor cells, most, if not all, of the hexokinase activity seems to be in a membrane-bound form.