FUNCTIONAL CONSEQUENCES OF NATURALLY-OCCURRING VARIANTS OF HUMAN HEXOKINASE II

Hexokinase II (HKII) catalyses a key step in glucose metabolism and ca n be regarded as a candidate gene for insulin resistance and type 2 (n on-insulin-dependent) diabetes mellitus. We observed previously four a mino acid substitutions among Finnish type 2 diabetic patients: Gln142 His, Ala314Val; 0.9 %, Arg353Cys; 2.7 % and Arg775Gln; 2.7 %. The Arg7 75Gln mutation was also observed in normal control subjects (2.1 %) an d the Gln142His substitution was found in both Type II diabetic and no rmal subjects with similar frequencies (similar to 20 %). Since Gin at position 142, Ala at 314 and Arg at 775 are present in human and rat hexokinases and could be important for structure and function of the e nzyme, we generated all four substitutions by site-directed mutagenesi s and expressed them in E. coli. None of these substitutions had any e ffect on HKII catalytic activity, K-m or V-max for glucose values in v itro. Thus unless these substitutions have an impact on enzyme activit y or regulation in vivo, it is unlikely that these substitutions contr ibute to the aetiology of Type II diabetes.