The specificity of human N-acetyl-beta-D-hexosaminidases towards glycosphincolipids is determined by an activator protein.

It has been very difficult to correlate, on the basis of in vitro measurements of substrate specificities, the glycosphingolipid storage patterns observed in different variants of infantile GM2 gangliosidosis with the hexosaminidase (hex) isoenzyme deficiencies underlying these diseases. However, the in vitro enzyme assays included detergents, which greatly enhanced the enzymic degradation of lipids by breaking down the large lipid micelles that cannot otherwise be attacked by the hydrolases. In vivo, the role of detergent is taken over by water-soluble, low molecular weight proteins, so-called activators, which bind the lipid monomers, thus solubilizing them. It can be shown that the activator protein for the enzymic degradation of ganglioside GM2 has a very strong preference for hex A over hex B; it also acts on glycolipid GA2 and, to a lesser extent, on kidney globoside. This isoenzyme specificity is much less prominent or even reversed when detergents are used to solubilize the substrates. The substrate specificities of hex A and hex B measured in the presence of sufficient amounts of the activator protein most probably reflect the conditions occurring in vivo. They can explain the lipid storage patterns observed in different variants of infantile GM2 gangliosidosis, especially the accumulation of ganglioside GM2 in variant B (where hex B is still present) and the reduced storage of GA2 in the same variant as compared to variants O and AB. The physiological significance of the activator protein is demonstrated in variant AB in which the activator is deficient, resulting in an accumulation of glycolipids GM2 and GA2.