Effect of Phospholipid Composition on the Surface Potential of Liposomes and the Activity of Enzymes Incorporated into Liposomes

Abstract

Microsomes of rat liver and brain and mitochondria of rat liver and guinea pig brown adipose tissue were solubilized with the nonionic detergent Lubrol-WX and the solubilized material was incorporated into liposomes of various phospholipid composition. In proteoliposomes thus formed the kinetics of arylsulfatase, glycerol-3-phosphate dehydrogenase, monoamine oxidase and acetylcholinesterase were measured. The apparent Km values of arylsulfatase and glycerol-3-phosphate dehydrogenase were higher in liposomes prepared with negatively charged phospholipids and lower in liposomes containing positively charged organic amines, as compared with the Km value of enzymes incorporated into liposomes prepared from phosphatidylcholine [PC] alone. The opposite was true for monoamine oxidase and acetylcholinesterase, i.e., enzymes possessing cationic substrates. Phospholipid composition did not essentially influence the activity of the enzymes extrapolated for infinite substrate concentration (V values). As compared with proteoliposomes made from PC, the binding constant (Ka) of 8-anilino-1-naphthalene sulfonate was higher when the vesicles contained acidic phospholipids or bis(hexadecanyl)phosphate and lower when they contained organic amines. A correlation between changes of the surface potential calculated from Ka values of anilinonaphthalene sulfonate and variations in apparent Km values of the 4 enzymes under investigation indicates that the activity of membrane-bound enzymes may be modulated by charged phospholipids due to decreasing or increasing substrate concentration in the unstirred layer, as predicted from the Boltzmann distribution.