A new variant of glucosephosphate isomerase (GPI) associated with hemolytic anemia, mental retardation, and muscular hypotonia is described. The defective enzyme showed increased affinity for fructose-6-phosphate (F-6-P), decreased affinity for glucose-6-phosphate (G-6-P) altered electrophoretic and isoelectrofocusing patterns, and shift to the left of the precipitin curve. The enzyme was stable under all the conditions tested (heat, urea, guanidine-HCl, and storage). Optimum pH, Ki for 6-phosphogluconic acid (6-PGA) and for erythrose-4-phosphate (E-4-P), molecular weight, GPI-related antigen concentration, immunodiffusion pattern, and immunoinactivation were in the normal range. This is the first example of the association of a stable mutant GPI with severe hemolytic anemia. Enzyme instability has been present in all previously reported cases.

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