ESTIMATION OF AVERAGE DEPTH OF PENETRATION OF MELANOTROPINS IN DIMYRISTOYLPHOSPHATIDYLGLYCEROL VESICLES

The interaction of alpha-melanocyte stimulating hormone (alpha-MSH) an d its analogs [Nle(4),D-Phe(7)]-alpha-MSH (MSH-I) and [Nle(4),Asp(5),D -Phe(7),Lys(10)]-alpha-MSH(4-10) (MSH-II) with vesicles of dimyristoyl phosphatidylglycerol (DMPG) was studied by steady-state fluorescence s pectroscopy. The association constants for the. interaction were obtai ned from binding isotherms, Electrostatic effects on the interaction w ere taken into account through calculation of Gouy-Chapman potentials. The quenching of fluorescence of the peptides by acrylamide and nitro xide labeled lipids demonstrated that insertion of the peptides into t he lipid phase of the vesicles causes the changes in the hormone's flu orescence in the presence of DMPG, The parallax method was employed fo r the estimation of an average depth of penetration of the peptides in the DMPG vesicles. It was found that the Trp residue in alpha-MSH and in MSH-II is positioned around the carbons 6 and 8 of the aliphatic c hain. The analog MSH-I goes deeper into the bilayer compared to the ot hers peptides, and the Trp residue locates between carbons 10 and 11 o f the acyl chain. The average depth of penetration shows correlation w ith the number of lipid molecules that interact with one molecule of p eptide. There is no direct correlation between the association constan ts for the lipid-peptide interactions and the depth of penetration of the hormone.