Involvement of arginine residues in catalysis by rat brain hexokinase

Inactivation of rat brain hexokinase (ATP:d-hexose 6-phosphotransferase, EC 2.7.1.1) by the arginine-specific reagent, phenylglyoxal, has been studied. Inactivation did not follow pseudo-first-order kinetics, suggesting the involvement of two or more arginine residues in catalytic function. Using [¹⁴C]phenylglyoxal, it was found that 5 of the 55 arginines per molecule of hexokinase react with this reagent, with an accompanying loss of over 90% of the catalytic activity. Virtually all of the activity loss occurs during derivatization of four relatively slower reacting arginines, with essentially no activity loss during derivatization of one rapidly reacting arginine. Inactivation by phenylglyoxal was not due to reaction with critical sulfhydryl groups in brain hexokinase since reactivity of the enzyme with the sulfhydryl reagent, 5,5′-dithiobis(2-nitrobenzoic acid) was not affected by prior treatment with phenylglyoxal. Comparison of amino acid composition, before and after reaction with phenylglyoxal, indicated that only the arginine content had been affected by phenylglyoxal treatment. The decrease in arginine content, measured by amino acid analysis, and the incorporation of phenylglyoxal, measured with [¹⁴C]phenylglyoxal, was consistent with the phenylglyoxal:arginine stoichiometry of 2:1 originally reported by K. Takahashi (1968, J. Biol. Chem.243, 6171–6179). Several ligands were tested and found to provide varying degrees of protection of hexokinase activity against phenylglyoxal. ATP and ADP alone provided only slight protection, but were highly effective in the presence of N-acetylglucosamine which itself gave only moderate protection. Glucose 6-phosphate and 1,5-anhydroglucitol 6-phosphate, both good inhibitors of brain hexokinase, were very effective while poorly inhibitory hexose 6-phosphates were not. Glucose was very effective, with protection afforded by other hexoses being correlated with their ability to serve as substrates (i.e., poor substrates also provided little protection against phenylglyoxal). The effectiveness of hexose 6-phosphates and hexoses in protecting the enzyme against inactivation by phenylglyoxal was related to their ability to induce conformational change in the enzyme. None of the ligands tested appreciably affected the reactivity of the rapidly reacting arginine residue. There was no correlation between the inhibition observed in the presence of various ligands and the number of arginines reacted with phenylglyoxal. The results were interpreted as indicating the involvement of two to four arginine residues in the catalytic function of brain hexokinase, possibly in the binding of anionic ligands such as ATP, ADP, or glucose 6-phosphate.