Serum beta-hexosaminidase isoenzymes are precursor forms

Abstract

The lysosomal enzyme beta-hexosaminidase (Hex, EC 3.2.1.52) was purified from human serum and placental tissue. On gel chromatography, Hex isoenzymes (Hex A, B and P) from serum showed a molecular weight of about 150 kDa, i.e. higher than the 120 kDa found for the placental (tissue) isoenzymes (Hex A and B). Sodium dodecyl sulphate polyacrylamide gel electrophoresis of non-reduced serum Hex A revealed two different subunits, denoted alpha and beta, corresponding to apparent molecular weights of 70 and 67 kDa, respectively. Serum Hex B and Hex P were composed of beta-subunits only. In contrast, the apparent molecular weights of the alpha- and beta-subunits originating from tissue were both 55 kDa. Reduction of tissue Hex A and Hex B resulted in cleavage of the beta-subunit into several fragments, whereas the beta-subunit in serum forms was resistant to this treatment. These results are consistent with the serum forms never having entered the lysosome prior to their release to the extracellular environment and serum forms can therefore be classified as precursor molecules. No mature (tissue) forms were found in serum. Isoelectric focusing before and after neuraminidase treatment of the serum isoenzymes showed that they all contained sialic acid and that Hex B and Hex P had an identical focusing pattern after desialylation. Serum beta-hexosaminidase is frequently elevated in liver disease, alcoholism and pregnancy and the results of the present study indicate that this is due to an increased synthesis and secretion of hypersialylated beta-subunits, rather than to release of intralysosomally stored enzyme.