A quantitative model for the dependence of solute permeability on peptide and cholesterol content in biomembranes

The influence of varying concentrations of a transmembrane peptide, gramicidin A (gA), and cholesterol (Chol) on the passive permeation of p-methylhippuric acid (MHA) and alpha-carbamoyl-p-methylhippuric acid (CMHA) across egg-lecithin membranes (EPC) has been investigated in vesicle efflux experiments. Incorporation of 0.25 volume fraction of gA in its nonchannel conformation increased the permeability coefficient (P-m) for CMHA by a factor of 6.0 +/- 1.8 but did not alter P-m for MHA, a more lipophilic permeant. In contrast, incorporation of 0.26 volume fraction Chol with no added protein decreased the P-m values for both CMHA acid MHA by similar factors of 4.2 +/- 1.1 and 3.5 +/- 1.2, respectively. A quantitative structure-transport model has been developed to account for the dependence of P-m, on the membrane concentrations of gA and Chol in terms of induced changes in both membrane chain ordering and hydrophobicity. Chain ordering is assumed to affect P-m for both permeants similarly since they are comparable in molecular size, while changes in P-m ratios in the presence of gA or Chol are attributed to alterations in membrane hydrophobicity. Changes in lipid chain ordering were detected by monitoring membrane fluidity using fluorescence anisotropy of 1-[4-(trimethylamino)phenyl]-6-phenylhex-1,3,5-triene incorporated into the membranes. The influence of additives on membrane hydrophobicity, which governs P-m ratios through effects on solute partitioning into the barrier domain, were rationalized within the framework of regular solution theory using solubility parameters as a measure of membrane hydrophobicity. Fits of the P-m ratios using the theoretical model yielded solubility parameters for gA and Chol in EPC membranes of 13.2 and 7.7 (cal/ml)(1/2), respectively, suggesting that gA decreases the barrier domain hydrophobicity while Chol has a minimal effect on barrier hydrophobicity. After correcting fbr barrier domain hydrophobicity, permeability decrements due to membrane ordering induced by gA or Chol were found to exhibit a strong correlation with membrane order as predicted by free-surface-area theory, regardless of whether gA or Chol is used as the ordering agent.