Effect of experimental conditions on the appearance of distinct forms of placental glucosylceramidase: use of gel filtration analysis as a means of ascertaining their occurrence

Studies on glucosylceramidase binding to phosphatidylserine liposomes: the role of bilayer curvature

The influence of phosphatidylserine (PS) liposome size on their capacity to activate and bind purified glucosylceramidase was investigated. Gel filtration and flotation experiments showed that large unilamellar vesicles (LUV) of either pure PS or PS in admixture with phosphatidylcholine (PC) are unable to tightly bind purified glucosylceramidase, and thus, to fully stimulate its activity. By contrast, small unilamellar vesicles (SUV) of PS adsorb glucosylceramidase can either be favoured or inhibited by factors affecting the bilayer curvature of PS liposomes. An increase of PS vesicle size induced by a fusogenic agent such as poly(ethylene glycol) (PEG), decreased enzyme binding and activity. On the contrary, the reduction of PS LUV size by sonication increased their stimulating ability. Enzyme association with PS SUV is reversible. In fact, glucosylceramidase bound to PS SUV was released from the lipid surface when the SUV were transformed into larger vesicles by PEG; dissociation from the vesicles resulted in a dramatic decrease of enzyme activity. Although PS LUV are unable to reconstitute glucosylceramidase, their association with oleic acid (OA) promotes the interaction with glucosylceramidase. This phenomenon is best explained in terms of OA-induced surface defects of PS LUV, with consequent exposure of the more hydrophobic part of the membrane and hence the improved binding of hydrophobic region/s of glucosylceramidase. Our data indicate that the physical organization of the PS-containing liposomes is of critical importance of glucosylceramidase reconstitution. The observation that physical changes of the lipid surface can markedly affect the enzyme activity offers a new approach to the study of glucosylceramidase regulation.

Reconstitution of glucosylceramidase on binding to acidic phospholipid-containing vesicles

Studies were conducted to investigate the mechanism by which acidic phospholipid-containing vesicles stimulate purified placental glucosylceramidase activity towards the water-soluble substrate 4-methylumbelliferyl-beta-D-glucopyranoside (MUGlc). Vesicles composed of pure phosphatidic acid (PA) or pure phosphatidylserine (PS) stimulated the activity of the enzyme about 20-fold. The inclusion of cholesterol and phosphatidylcholine (PC), beside PA, into the vesicles slightly improved their stimulatory effect. Further addition of oleic acid (OA) markedly increased the stimulation (50-fold). By ultracentrifugation and gel permeation procedures it was shown that, under optimal conditions for stimulation of the MUGlc hydrolysis by acidic phospholipid-containing vesicles, purified glucosylceramidase spontaneously binds to their surface. Interestingly, the molar fraction of the acidic phospholipid into the mixed vesicles, rather than its concentration in the assay, is the crucial parameter for activation and binding of the enzyme. The importance of glucosylceramidase association with appropriate vesicles for enzyme activation was indicated by observing that the presence of 0.2 M citrate-phosphate buffer (pH 5.5), that prevented the binding to PA-containing surfaces, also inhibited the enzyme activity. Our results indicate that the reconstitution of glucosylceramidase activity occurs through the spontaneous tight association of the enzymatic protein with preformed acidic phospholipid-containing vesicles.