Human hexosaminidase isozymes: chromatographic separation as an aid to heterozygote identification

The correct identification of Tay-Sachs heterozygotes requires a reliable procedure for separation and quantiation of the hexosaminidase isozymes. The most commonly employed method involves thermal inactivation of the heat labile hexosaminidase A assay of residual enzyme activity. This procedure, however, consistently yields a significantly lower absolute and relative activity of hexosaminidase A and a higher activity of the thermostable components (B and I) in comparison with the results obtained by DEAE-cellulose chromatography. DEAE-cellulose chromatographic separation of the hexosaminidase isozymes in serum following thermal inactivation reveals the presence of relative and absolute increase in the activity of the B and I components in addition to loss of the heat-labile A isozyme. Because the conversion of hexosaminidase A into thermostable forms by heating may vary according to the conditions employed, the thermal inactivation procedure may lead to ambiguity in heterozygote identification. This difficulty can be minimized by fractionation of the hexosaminidase isozymes by DEAE-cellulose chromatography followed by assay of the individual components. In addition to the Tay-Sachs carrier state, other conditions can alter the distribution of the hexosaminidase isozymes in tissues and body fluids. For example in serum of patients with juvenile diabetes mellitus there is a characteristic elevation of hexosaminidase B and less consistently, of hexosaminidase A. Since the activity of hexosaminidase A in serum of diabetics fractionated by ion exchange chromatography is at least as high as the activity in serum of healthy non-carriers, patients with diabetes can be easily differentiated from Tay-Sachs heterozygotes. Similarly, the distribution of the hexosaminidase isozymes in serum is altered during pregnancy, where there is usually a significant rise in hexosaminidase A and I (P). However, during pregnancy activities of hexosaminidase A and I in serum of obligate Tay-Sachs carriers are only 50% of the values observed in non-carriers at comparable gestational periods. Since the absolute activities of hexosaminidase A in serum of pregnant carriers obtained by ion exchange chromatography do not overlap with the activities in serum of non-carrier pregnant women at comparable gestational periods, this method has obvious advantages for identification of pregnancies where the fetus may be at risk for Tay-Sachs disease.