Interaction of saposins, acidic lipids, and glucosylceramidase.

Activity of lysosomal glucosylceramidase is stimulated by two small glycoproteins, saposin A and C, which are, together with two other similar glycoproteins, derived from a single precursor protein. This enzyme is also stimulated by naturally occurring acidic lipids, such as phosphatidylserine and gangliosides. Using highly purified glucosylceramidase, saposins, and acidic lipids, the mechanism of enzyme stimulation was studied by investigating complex formation between the three components and by examining effects on activity caused by changing amounts of saposins and acidic lipids, individually or in combination. The results indicated that acidic lipids form a water-soluble complex with glucosylceramidase but not with saposins and that saposins and acidic lipids each bind to the enzyme at two different sites for the activation. Based on these observations, the previously proposed three-binding sites model of glucosylceramidase, activator, and substrate was modified to one composed of four binding sites: one for carbohydrate of the substrate, one for aglycon, one for acidic lipids, and one for saposins.