Purification and properties of the hexosaminidase A-activating protein from human liver

[摘要]

Human liver extracts contain an activating protein which is required for hexosaminidase A-catalysed hydrolysis of the N-acetylgalactosaminyl linkage of G_M2 ganglioside [N-acetylgalactosaminyl-(N-acetylneuraminyl) galactosylglucosylceramide]. A partially purified preparation of human liver hexosaminidase A that is substantially free of G_M2 ganglioside hydrolase activity is used to assay the activating protein. The proceudres of heat and alcohol denaturation, ion-exchange chromatography and gel filtration were used to purify the activating protein over 100-fold from crude human liver extracts. When the purified activating protein is analysed by polyacrylamide-gel disc electrophoresis, two closely migrating protein bands are seen. When purified activating protein is used to reconstitute the G_M2 ganglioside hydrolase activity, the rate of reaction is proportional to the amount of hexosaminidase A used. The activation is specific for G_M2 ganglioside and and hexosaminidase A. The activating protein did not stimulate hydrolysis of asialo-G_M2 ganglioside by either hexosaminidase A or B. Hexosaminidase B did not catalyse hydrolysis of G_M2 ganglioside with or without the activator. Kinetic experiments suggest the presence of an enzyme–activator complex. The dissociation constant of this complex is decreased when higher concentrations of substrate are used, suggesting the formation of a ternary complex between enzyme, activator and substrate. Determination of the molecular weight of the activating protein by gel-filtration and sedimentation-velocity methods gave values of 36000 and 39000 respectively.