Pyruvate kinase Greensboro. A four-generation study of a high K0.5s (phosphoenolpyruvate) variant

The proband with lifelong hemolytic anemia has a high K0.5s phosphoenolypyruvate (PEP) erythrocyte pyruvate kinase (PK) variant substantially but incompletely normalized by the allosteric modifier fructose-1,6-diphosphate (F-1,6-P2) with conversion of sigmoidal to hyperbolic kinetics. Heterozygotes in four generations express qualitatively identical but less severely abnormal kinetics and lack overt hemolysis. Kinetic abnormalities are closely mimicked by sulfhydryl modification of normal PK. Three distinct clinical and metabolic phenotypes characterize the proband and two sisters: variant PK and hemolytic anemia, variant PK without clinical manifestations or hemolysis, and complete normality. Their mother, whose red cell PK is entirely normal except for a questionably slightly low Vmax, is postulated to express the gene products of nonidentical alleles, one encoding a product with mildly less favorable catalytic characteristics. At low PEP concentrations, the proband and heterozygotes for the PK mutant express only a very small fraction of normal PK activity despite apparent inheritance of one normal allele in the latter. Evidence suggests that disproportionately lowered PK activity may be a property of a heterotetrameric PK. Illusory abnormalities in nucleotide specificity are artifacts of diminished substrate affinity characterizing the mutant PK.