To determine if sequence variants in the glucokinase (GCK) gene contribute to the high risk of impaired glucose metabolism in Japanese-Americans and whether the gene sequence differs between Japanese-Americans and Caucasians.

Forty-seven unrelated Japanese-Americans with one or more first-degree relatives with non-insulin-dependent diabetes mellitus (NIDDM) were selected, irrespective of glucose tolerance status. By World Health Organization criteria, 13 had normal glucose tolerance, 11 had impaired glucose tolerance, and 23 had NIDDM. Variations in the GCK gene were identified by single-strand conformation polymorphism analysis and sequenced using standard techniques.

Six variants of the GCK gene were identified in a total of 21 subjects: 1) a G→A substitution at nucleotide −30 in the β-cell-specific promoter; 2) an A→G substitution at nucleotide 244 in the 5'-untranslated region (5'-UTR) of exon 1a; 3) a C→G substitution at nucleotide 403 in the 5'-UTR of exon 1a; 4) a G→A variant 13 base pair (bp) 5' to the intron 3 exon 4 junction; 5) a silent substitution in the third base of codon 145 in exon 4; and 6) a C→T substitution 8 bp 3' to the exon 9 intron 9 junction. None of these variations would be expected to affect the structure of the GCK enzyme. While none of these variants were significantly associated with IGT or NIDDM, a nonsignificant increase in the β-cell promoter variant was observed in subjects with abnormal glucose tolerance. No uniform sequence differences in the GCK gene were identified between Japanese-American and Caucasian-American subjects.

Mutations affecting the amino acid sequence of GCK do not account for the increased incidence of impaired glucose metabolism in Japanese-Americans, and the gene sequence does not uniformly differ from that in Caucasians.