Genetic heterogeneity within an electrophoretic phenotype of phosphoglucose isomerase in a Japanese population

The thermostability of the five kinds of electrophoretically variant phenotypes of GPI which were found in Japanese in a previous study (Tanis et al. 1977) was examined. The most frequently found variant phenotype, termed GPI 1-4HIR1, observed in 20 individuals could be divided into three distinct classes on the basis of thermostability characteristics. These classes were termed 'stable', 'labile', and 'very labile'. 'Stable' lost approximately 20 and 60% of its original activity after heating 10 and 30 min at 52-5 degrees C, respectively, while normal GPI lost approximately 30 and 70% of its original activity. 'Labile' lost approximately 40 and 80%, and 'very labile' lost approximately 55 and 90% of its original activity under the same heating conditions. Electrophoresis showed that that thermostability was a characteristic of the variant protein molecule but not of the electrophoretically normal molecule. The order of the stability of these three kinds of variants against 5 M urea was the same as that of their thermostability. No difference against inhibition by 6-phosphogluconate was observed among the normal and the variant phosphoglucose isomerases. Family studies confirmed the genetic nature of the thermo- and urea stability differences among the affected individuals.