Isolation and relationship of human hexosaminidases.

A method for the preparation of highly purified @-hexosaminidase isozymes A and B from human placenta is presented. The purified enzymes possess identical molecular weight and exhibit similar kinetic parameters with artificial fluorogenic substrates. Both enzymes catalyze the hydrolysis of Tay-Sachs ganglioside (Cer-Glc-Gal(NeuAc)-GalNAc) and the corresponding asialo-derivative (Cer-Glc-GalGalNAc). Both enzymes are composed of four subunits with a mass of 33,000 daltons each. The enzymes seem to differ only in the interactions of these subunits to form the fully associated enzyme by hydrophobic association and the formation of disulfide bonds. It is possible to convert hexosaminidase A into hexosaminidase B by heating under carefully controlled conditions. The newly formed B has the ion exchange properties and subunit interactions characteristic of highly purified hexosaminidase B isolated from fresh tissue. This conversion does not depend on the removal of N-acetylneuraminic acid. Thus, the two hexosaminidases seem to exist as conformers of each other. A model for the relationship of hexosaminidases in normal tissues and in the various clinical conditions which are characterized by the absence of one or more of these enzymes (Tay-Sachs disease and its variant forms) is presented. A benign form of hexosaminidase A deficiency is postulated based on the ability of both A and B to catalyze the hydrolysis of Tay-Sachs ganglioside.