Involvement of phospholipids in the modulation of a membrane-bound brain fucosyltransferase.

Abstract

Microsomal fucosyltransferase isolated from sheep brain is strongly enhanced by charged lysophospholipids such as lysophosphatidylinositol and lysophosphatidic acid, while the corresponding phospholipids are inhibitive. Lysophosphatidylcholine (lyso-PC) also greatly increases the enzymatic activity and leads to its solubilization. Its stimulatory effect is related to the length of the fatty acyl chain involved in the lyso-PC structure: fatty acids C18 and C20 are less activating than the fatty acids C14-C16. Stimulation is restored when C18 fatty acids are unsaturated (e.g., C18:1-C18:3). Enzymatic activity enhancement is decreased when phosphatidylcholine structures are reformed by the addition of lyso-PC and the corresponding fatty acid. The physical state of these structures has no influence. These data provide evidence that bilayer structures do not modify enzymatic activity, while micellar structures formed by detergents and lysophospholipids lead to a strong increase in fucosyltransferase activity. However, lyso-PC does not interact in exactly the same way as Triton X-100. Although they both enhance the maximal velocity of fucosyltransferase for its two substrates, GDP-fucose and asialofetuin, the effect with lyso-PC is greater, and it clearly enables a better affinity for GDP-fucose. Endogenous phospholipids are also able to modify enzymatic activity. Hydrolysis of phosphatidylcholine by phospholipase A2 leads to an enzymatic stimulation.