Interrelationship of hexosaminidases A and B: conformation of the common and the unique subunit theory.

Human kidney hexosaminidase A (beta-N-acetylglucosaminidase; 2-acetamido-2-deoxy-beta-D-glucoside acetamidodeoxyglucohydrolase; EC 3.2.1.30) is a heteropolymer of two immunologically distinct subunits designated as alpha and beta. Hexosaminidase B, however, is a homopolymer comprised entirely of beta subunits. When human kidney hexosaminidase A was dissociated into its subunits by p-hydroxymercuribenzoate, three distinct proteins having isoelectric points of pH 7.2.5.4, and 4.3 were isolated. The fraction having an isoelectric point of pH 7.2, designated as beta fraction, was electrophoretically and immunologically identical to hexosaminidase B and was enzymatically active. The proteins having isoelectric points of pH 5.4 and 4.3, designated as hexosaminidase Ai and alpha fractions, respectively, were enzymatically inactive and crossreacted with antiserum against hexosaminidase A and not with antiserum against hexosaminidase B. Upon incubation of p-hydroxymercuribenzoate-treated hexosaminidase A with dithiothreitol,, hexosaminidase A activity, as well as antigenicity, was regenerated, indicating that alpha and beta subunits hybridize to form hexosaminidase A. Antibodies raised in rabbits against beta fractions reacted with both hexosaminidase A and B, whereas the antibodies against alpha and hexosaminidase Ai fractions reacted only against hexosaminidase A. This would indicate that both fractions are composed only of subunits unique to hexosaminidase A. The molecular weights of alpha,beta, and hexosaminidase Ai fractions were estimated to be 47,000, 120,000, and 180,000 respectively, by Sephadex gel filtration. Upon urea-sodium dodecyl sulfate polyacrylamide electrophoresis, each of the three fractions dissociated into a single polypeptide having a molecular weight of approximately 18,000. It is concluded that p-hydroxymercuribenzoate dissociates hexosaminidase A, (alphabeta)3, into its subunits, and the beta subunits can reassociate to form relatively stable hexosaminidase B, (betabeta)3, while the alpha subunits reassociate in both the dimeric state, alpha2, and a polymeric state, alpha8.