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 (Pm) for CMHA by a factor of 6.0 +/- 1.8 but did not alter Pm for MHA, a more lipophilic permeant. In contrast, incorporation of 0.26 volume fraction Chol with no added protein decreased the Pm values for both CMHA and 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 Pm 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 Pm for both permeants similarly since they are comparable in molecular size, while changes in Pm 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-phenylhexa-1,3,5-triene incorporated into the membranes. The influence of additives on membrane hydrophobicity, which governs Pm 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 Pm 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 for 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.