A deletion mutation in glucosephosphate isomerase (GPI Denton).

A new genetic variant form of glucosephosphate isomerase has been found in a family heterozygous for the mutant allele. The mutant enzyme, unlike other phenotypic variants, does not appear to be the result of a single amino acid replacement. The allozyme exhibits an isoelectric point of 5.7 and is thus much more acidic than the normal enzyme (pI = 9.3). The allozyme has been isolated from placenta and separated from the normal homodimer and heterodimer by isoelectric focusing. The enzyme exhibits normal Km and Ki values for the substrates and competitive inhibitors. The allozyme exhibits a normal pH optimum and thermal stability. However, the molecular specific activity of the variant enzyme as quantitated by radioimmunoassay is significantly lower than normal. Analytical gel filtration revealed that the molecular weight of the weight of the enzyme is significantly lower than the normal enzyme. These data thus suggest that the phenotype is unlike any previously reported and is due to a deletion mutation.