Characterization of the association of Electrophorus electricus acetylcholinesterase with sphingomyelin liposomes. Relevance to collagen-sphingomyelin interactions.

Electrophorus electricus acetylcholinesterase is a large polymorphic enzyme. Its native forms 18 S, 14 S and 8.5 S possess a tail having a collagen-like structure. It was suggested that this tail is involved in the anchorage of the enzyme at the terminal of the synapse. Watkins et al. [1] showed that all forms of the enzyme having a collagen segment also bind to sphingomyelin liposomes with almost no binding to phosphatidylcholine (PC) liposomes. In agreement with the above results, the binding of acetylcholinesterase reported here was independent of the following liposomal parameters (a) curvature, (b) the physical state of the bilayer, (c) the gel to liquid crystalline phase transition of sphingomyelin, (d) stereospecificity of the sphingomyelin, (e) acyl chain of the sphingomyelin. The binding was reduced with increasing PC content in sphingomyelin vesicles. The binding has no effect on the bilayer integrity. The enzymatic activity can be released from the vesicles by incubation with collagenase. The association of the enzyme with the liposomes had minimal effect on its kinetic parameters (Km, Vmax). The only detectable effect was increasing enzyme stability at low enzyme concentration. This suggested that the binding of the enzyme to sphingomyelin liposomes reduced its surface denaturation. Such association was not unique to acetylcholinesterase since collagen showed similar behavior. Collagen binding to sphingomyelin liposomes was 5-10-times larger than to PC liposomes. The exact details of the interaction of collagen and collagen-like peptides with sphingomyelin bilayers are yet unknown although it differs from the well documented hydrophobic or electrostatic interactions [7]. This work proposes hydrogen bonding as a third mechanism which involves the interface region of sphingolipids molecules and the collagen or collagen-like tail of acetylcholinesterase. This binding is also of interest due to its correlation to the accumulation of sphingomyelin and collagen during aging and the development of atherosclerosis in blood vessels of mammals.