Nonenzymatic conversion of human hexosaminidase A

Incubation of purified hexosaminidase A with merthiolate, parahydroxymercuribenzoic acid, or silver ions resulted in the formation of an enzyme which was identical, in all respects tested, with hexosaminidase B. Its electrophoretic mobility was identical to hexosaminidase B at three different pH levels. Its chromatographic properties, thermostability, and immunologic reactivity with specific anti-hexosaminidase A and anti-hexosaminidase B antisera were indistinguishable from hexosaminidase B. Conversion could be prevented by GSH but was not reversed by GSH once it had occurred. Conversion of hexosaminidase A to hexosaminidase B was accompanied by the appearance of an electrophoretically rapid, catalytically inactive protein. Hexosaminidase B, itself, was unaltered by incubation with merthiolate. These findings support a previously proposed model of hexosaminidase structure in which hexosaminidase A is (αβ)₃ and hexosaminidase B is (ββ)₂. On treatment of hexosaminidase A with merthiolate and other converting agents α and β subunits are presumably dissociated and they reassociate as (ββ)₃, that is, hexosaminidase B. The expected free or polymerized, catalytically inactive, α chains are detected on acrylamide gel electrophoresis. We suggest that the catalytic site of human hexosaminidase may be present only on the β subunit and that the α subunit influences the substrate specificity of the enzyme, particularly as directed toward GM₂.