Cholesterol superlattice modulates the activity of cholesterol oxidase in lipid membranes

Here, the interplay between membrane cholesterol lateral organization and the activity of membrane surface-acting enzymes was addressed using soil bacteria cholesterol oxidase (COD) as a model. Specifically, the effect of the membrane cholesterol mole fraction on the initial rate of cholesterol oxidation catalyzed by COD was investigated at 37 degreesC using cholesterol/l-palmitoyl-2-oleoyl-L-alpha-phosphatidylcholine (POPC) large unilamellar vesicles (LUVs, similar to800 nm in diameter). In the three concentration ranges examined (18.8-21.2, 23.6-26.3, and 32.2-34.5 mol % cholesterol), the initial activity of COD changed with cholesterol mole fraction in a biphasic manner, exhibiting a local maximum at 19.7, 25.0, and 33.4 mol %. Within the experimental errors, these mole fractions agree with the critical cholesterol mole fractions (C-r) (20.0, 25.0, and 33.3) theoretically predicted for maximal superlattice formation. The activity variation with cholesterol content was correlated well with the area of regular distribution (A(reg)) in the plane of the membrane as determined by nystatin fluorescence. A similar biphasic change in COD activity was detected at the critical sterol mole fraction 20 mol % in dehydroergosterol (DHE)/POPC LUVs (similar to168 nm in diameter). These results indicate that the activity of COD is regulated by the extent of sterol superlattice for both sterols (DHE and cholesterol) and for a wide range of vesicle sizes (similar to 168 - 800 nm). The present work on COD and the previous study on phospholipase A(2) (sPLA(2)) [Liu and Chong (1999) Biochemistry 38, 3867-3873] suggest that the activities of some surface-acting enzymes may be regulated by the extent of sterol superlattice in the membrane in a substrate-dependent manner. When the substrate is a sterol, as it is with COD, the enzyme activity reaches a local maximum at C-r. When phospholipid is the substrate, the minimum activity is at C-r, as is the case with sPLA(2). Both phenomena are in accordance with the sterol superlattice model and manifest the functional importance of membrane cholesterol content.