Unique phenotypic expression of glucosephosphate isomerase deficiency

Studies of a Mexican kindred present evidence for a unique phenotype of erythrocyte glucosephosphate isomerase, GPI Valle Hermoso. The proband was apparently the homozygous recipient of a mutant autosomal allele governing production of an isozyme characterized by decreased activity, marked thermal instability, normal kinetics and pH optimum, and normal starch gel electrophoretic patterns. Unlike previously known cases, leukocyte and plasma GPI activities were unimpaired. This suggested that the structural alteration primarily induced enzyme instability without drastically curtailing catalytic effectiveness, thereby allowing compensation by cells capable of continued protein synthesis. Age-related losses of GPI, however, were not evident by density-gradient fractionation of affected erythrocytes.