Substrate binding properties of the human liver hexosaminidase A activator protein

The human liver hexosaminidase A activator protein has been shown to bind to the substrate GM2 ganglioside by cosedimentation in sucrose density gradients. Among other proteins tested only serum albumin forms a GM2 ganglioside - protein complex. Both activator protein and albumin bind to the monomeric form of GM2 ganglioside and not to the micellar form of the substrate. The GM2 ganglioside - activator protein complex can be recovered in a stable form. Storage at various temperatures or incubation with monosaccharides or with detergent does not result in dissociation of the complex. GM2 ganglioside in the activator-substrate complex is exchangeable with exogenous GM2 ganglioside. Hexosaminidase A, prepared from human liver, hydrolyzes GM2 ganglioside in the activator-substrate complex as efficiently as GM2 ganglioside supplied exogenously. The activator - GM2 ganglioside complex forms at pH 3.0 and exhibits an optimum similar to the pH optimum of hexosaminidase A catalyzed hydrolysis of GM2 ganglioside in the presence of the activator; however, the ability of the activator to stimulate enzymic hydrolysis of substrate is rapidly lost after heating at 75 degrees C, whereas its ability to bind substrate is increased. The sphingolipids cerebroside sulfate and sphingomyelin show little or no binding to the hexosaminidase A activator protein nor do they inhibit activation of hexosaminidase A catalyzed hydrolysis of GM2 ganglioside. By contrast GM1 ganglioside inhibits both substrate binding and enzyme activation.