A rare genetically determined electrophoretic variant of human leucocyte type III hexokinase.

Various lines of evidence from starch gel electrophoretic experiments demonstrate the existence of a genetically determined rare variant form of the type III isozyme of hexokinase (HK) in the leucocytes of a small percentage of the general human population. This enzymatically active variant (designated IIIS) migrates slightly, but significantly, slower than the common form (designated IIIF). In addition to finding various individuals with a two-banded pattern (heterozygotes containing both IIIS and IIIF), a finding reported previously by S. Povey, G. Corney, and H. Harris ((1975) Ann. Hum. Genet. 38, 407-415), we discovered one person homozygous for the variant phenotype. In close agreement with Povey et al., screening of 59 individuals at random indicated a gene frequency of about 0.017 for the IIIS allele, corresponding to a homozygous genotype for this allele that would be expected in about one of every 3500 individuals. Experiments involving the mixing of blood samples from the individual homozygous for IIIS with those homozygous for IIIF indicate that secondary in vitro changes, a possibility suggested by Povey et al., are not responsible for the appearance of the variant. This conclusion was supported by a demonstration of the specificity of the alteration in type III's mobility in comparison with the lack of alterations in any of the LDH isozymes, glucose-6-phosphate dehydrogenase, and various amido black-stainable proteins. These studies confirm the proposal for a genetically determined polymorphism of type III HK. No differences could be found between the total HK activity (according to spectrophotometric assays) of extracts from the subject homozygous for the variant and the activity from the homozygote for the common form, in terms of either their Km values for glucose or their heat stability properties. The similarity of Km values was supported by kinetic assays performed during staining of the individual forms on electrophoretic gels. Previous findings, reported elsewhere, of type III HK in RBC extracts were shown here to be attributable to contamination, by leucocytes, of the extracts. As a consequence of these studies, slight, but significant, amounts of type II-like HK were also discovered in leucocytes. Because our studies described above were completed in 1969, advantage was taken of the opportunity to test the HK pattern 17 years later from some of the same subjects. The patterns of the homozygotes for IIIS and for IIIF and the heterozygotes were found to be identical to the original ones, indicating no age-, environmental-, or other time-related changes that could explain the variation in type III HK.