AVAILABILITY FOR ENZYME-CATALYZED OXIDATION OF CHOLESTEROL IN MIXED MONOLAYERS CONTAINING BOTH PHOSPHATIDYLCHOLINE AND SPHINGOMYELIN

In this study we have examined the interaction between cholesterol and phospholipids in monolayers using cholesterol oxidase (Streptomyces cinnamomeus) as a probe. Monolayers containing cholesterol and phospholipids in different molar ratios were exposed to cholesterol oxidase at a lateral surface pressure of 20 mN/m (at 30 degrees C). The rate of cholesterol oxidation by cholesterol oxidase was faster in a monolayer consisting of a mono-unsaturated phospholipid (either 1-stearoyl-2-oleoyl-sn-glycero-3-phospho (SOPC) or N-oleoyl-sphingomyelin (O-SPM)) and cholesterol than it was in a monolayer of a saturated phospholipid (either 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) or N-stearoyl-sphingomyelin (S-SPM)) and cholesterol. This suggests that the susceptibility of cholesterol to oxidation by cholesterol oxidase was markedly affected by the phospholipid acyl chain composition. In addition, cholesterol was oxidized more readily in a phosphatidylcholine-containing monolayer as compared with a sphingomyelin monolayer (at a similar degree of acyl chain saturation). The average rate of oxidation, as a function of the cholesterol/phospholipid (C/PL) molar ratio in a binary monolayer (with cholesterol and one phospholipid class), was linear except for one discontinuity, at 1:1 for phosphatidylcholine monolayers (either SOPC or DSPC) and at 2:1 for sphingomyelin monolayers (O-SPM or S-SPM). We interpret these discontinuities as indicating the stoichiometry at which cholesterol can exist dispersed in the monolayer without lateral segregation into cholesterol-rich clusters. Next, ternary monolayers were examined (with cholesterol and one phosphatidylcholine and one sphingomyelin species). In ternary monolayers where sphingomyelin dominated (67 mol% sphingomyelin and 33 mol% phosphatidylcholine plus cholesterol) the stoichiometry was 2:1 irrespective of the acyl chain composition of the phospholipid species. In ternary monolayers where phosphatidylcholine dominated (67 mol% phosphatidylcholine and 33 mol% sphingomyelin) the stoichiometry was 1:1 for monolayers containing SOPC/S-SPM, SOPC/O-SPM and DSPC/S-SPM but 2:1 for the system DSPC/O-SPM. A further titration of the DSPC/O-SPM system showed that the 2:1 stoichiometry changed to a 1:1 stoichiometry only when the O-SPM content decreased below 5 mol%.