SERUM BETA-HEXOSAMINIDASE ISOENZYMES ARE PRECURSOR FORMS

The lysosomal enzyme beta-hexosaminidase (Hex, EC 3.2.1.52) was purifi ed from human serum and placental tissue. On gel chromatography, Hex i soenzymes (Hex A, B and P) from serum showed a molecular weight of abo ut 150 kDa, i.e. higher than the 120 kDa found for the placental (tiss ue) isoenzymes (Hex A and B). Sodium dodecyl sulphate polyacrylamide g el electrophoresis of non-reduced serum Hex A revealed two different s ubunits, denoted alpha and beta, corresponding to apparent molecular w eights of 70 and 67 kDa, respectively. Serum Hex B and Hex P were comp osed of beta-subunits only. In contrast, the apparent molecular weight s 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 w ith the serum forms never having entered the lysosome prior to their r elease to the extracellular environment and serum forms can therefore be classified as precursor molecules. No mature (tissue) forms were fo und in serum. Isoelectric focusing before and after neuraminidase trea tment of the serum isoenzymes showed that they all contained sialic ac id and that Hex B and Hex P had an identical focusing pattern after de sialylation. Serum beta-hexosaminidase is frequently elevated in liver disease, alcoholism and pregnancy and the results of the present stud y indicate that this is due to an increased synthesis and secretion of hypersialylated beta-subunits, rather than to release of intralysosom ally stored enzyme.