HUMAN HEXOKINASE II GENE - EXON-INTRON ORGANIZATION, MUTATION SCREENING IN NIDDM, AND ITS RELATIONSHIP TO MUSCLE HEXOKINASE-ACTIVITY

In muscle, hexokinase II (HK2) regulates phosphorylation of glucose to glucose 6-phosphate, which has been reported to be impaired in patien ts with non-insulin-dependent diabetes mellitus (NIDDM), Here we repor t decreased HK2 enzyme activity in skeletal muscle biopsies from patie nts with impaired glucose tolerance compared with healthy control subj ects (2.7 +/- 0.9 vs 4.9 +/- 1.1 nmol . min(-1). mg protein(-1)). Ther efore, mutations in the HK2 gene could contribute to skeletal muscle i nsulin resistance in NIDDM. To address this question, we first determi ned the exon-intron structure of the human HK2 gene and using this inf ormation, we screened all 18 exons with single-strand conformation pol ymorphism technique in 80 Finnish NIDDM patients. Nine nucleotide subs titutions were found, one of which was a missense mutation (Gln(142)-H is(142)) in exon 4. In human muscle, a single HK2, mRNA transcript wit h a size of approximately 5500 nucleotides was detected with Northern blot analysis. We also describe an HK2 pseudogene (HK2P1), which was m apped to chromosome 4, band q26, by fluorescence in situ hybridization to metaphase chromosomes. The clinical characteristics and HK2 enzyme activities of the subjects with either Gin or His at residue 142 did not differ from each other. Instead, HK2 activity correlated inversely with fasting blood glucose levels, suggesting that changes in HK2 act ivity could be secondary to other metabolic abnormalities (r = 0.55; p < 0.0003; n = 39). In conclusion; the data suggest that impaired HK2 activity in prediabetic individuals is a consequence of impaired gluco se tolerance rather than of a genetic abnormality. The data thus seem to rule out mutations in the HK2 gene as a major cause of inherited in sulin resistance in NIDDM.