Comparison of Triton X-100 penetration into phosphatidylcholine and sphingomyelin mono- and bilayers

This study has addressed the question whether there is a difference in resi stance to Triton X-100 penetration of sphingomyelin and phosphatidylcholine membranes. We have studied this by measuring the penetration of Triton X-1 00 into monolayers of either D-erythro-N-palmitoylsphingomyelin or dipalmit oylphosphatidylcholine (DPPC) at constant lateral surface pressure and 22 d egreesC. In addition, we also compared how Triton X-100 penetrated into egg phosphatidylcholine and D-erythro-24:1(Delta 15c) sphingomyelin monolayers and how the presence of cholesterol affected the penetration process. In b ilayer membranes we assessed Triton X-100 insertion and solubilization by s tudying the thermodynamics of the partition process and determining the det ergent:ratio at which the solubilization is initialized. The penetration of Triton X-100 from the subphase into the monolayers was concentration and s urface pressure dependent. The penetration increased linearly with time and concentration for DPPC monolayers, but with sphingomyelin monolayers, the response was clearly biphasic and faster. If the penetration was allowed to proceed, the monolayers eventually became unstable and collapsed (at colla pse the detergent-to-lipid ratio was about 1:4 in the monolayer). Inclusion of 30 mol % cholesterol in the monolayers made them more resistant to Trit on X-100 penetration, irrespective of the host phospholipid type. In bilaye rs the partition coefficient (K) becomes lower and the enthalpy change (Del taH) higher for partitioning of Triton X-100 into D-erythro-N-palmitoylsphi ngomyelin as compared to into DPPC. Onset of solubilization was reached at lower detergent:lipid ratios in D-erythro-N-palmitoylsphingomyelin than in DPPC vesicles. In conclusion, we have observed that D-erythro-N-palmitoylsp hingomyelin monolayers; do not show increased resistance toward Triton X-10 0 penetration as compared with membranes prepared from an acyl-matched glyc erophospholipid, at least not at room temperature and not even when cholest erol is present. Sphingomyelin bilayers are more susceptible to Triton X-10 0 solubilization than phosphatidylcholine bilayers, although K was lower fo r Triton X-100 into sphingomyelin bilayers.