Nonuniform deficiency of hexosaminidase a in tissues and fluids of two unrelated individuals

Serum samples from two unrelated, clinically normal individuals lacked detectable hexosaminidase A by heat inactivation and electrophoretic analysis. In contrast, 15 and 17% of the hexosaminidase in their leukocytes and 23 and 26% of the hexosaminidase of their cultured fibroblasts had the heat stability and electrophoretic properties of the A form of this enzyme. An in vitro measurement of fibroblasts GM₂ ganglioside-β-galactosaminidase was in the range expected for Tay-Sachs disease (TSD) heterozygotes (2.5 and 3.1 versus a normal mean of 3.7). In contrast, fibroblasts from a patient with TSD, analyzed in an identical fashion, contained no detectable activity. Ten days after addition of labeled GM₂ ganglioside to the medium of the cultured fibroblasts, 43 and 59% of the radioactivity taken up by the cells of these patients remained as unhydrolyzed ganglioside as compared with 94% in TSD fibroblasts and 42% in control cells. An analysis of sphingolipid composition by high performance liquid chromatography although the endogenous level of GM₂ was elevated in TSD fibroblasts (0.39 nmoles/mg protein) there was no increase in the cells of these patients (0 and 0.12 versus control of 0.17 nmoles/mg protein). Finally, the synthesis of hexosaminidase was examined by an electrophoretic analysis of immunoprecipitates of the enzyme precursors that had been radiolabeled by culturing fibroblasts in medium containing [³H]-leucine. These studies revealed a normal pattern of biosynthesis, processing and secretion of the α and β chains. The ratio of the α chain to the β chain, however, was in the range expected for TSD heterozygotes. Speculation: In all likelihood, these individuals suffer from a genetic or physiologic abnormality that results in the absence of hexosaminidase A in serum and a relative preservation of activity against both GM2 ganglioside and 4-methylumbelliferyl-N-acetyl-y3-glu- cosaminide in cultured fibroblasts.