CHOLESTEROL CONDENSATION OF ALPHA-LINOLENIC AND GAMMA-LINOLENIC ACID-CONTAINING PHOSPHATIDYLCHOLINE MONOLAYERS AND BILAYERS

Cholesterol is demonstrated to condense phosphatidylcholine (PC) monol ayers and bilayers containing stearic acid in the sn-1 position and al pha-linolenic acid in the sn-2 position (18:0,alpha-18:3 PC) but has n o effect when gamma-linolenic acid occupies the sn-2 position (18:0,ga mma-18:3 PC). Cholesterol-induced condensation is measured by area/mol ecule determinations made on monolayers using a Langmuir trough, while condensation in bilayers is followed by the fluorescent dyes merocyan ine (MC540) and dansyllysine. Permeability to erythritol is also demon strated to be diminished by cholesterol for the condensable 18:0,alpha -18:3 PC bilayer membranes but not the 18:0,gamma-18:3 PC membranes, a lpha- and gamma-linolenic acid are isomers containing 18 carbons and t hree unsaturations. Both fatty acids have unsaturations at positions 9 and 12 and differ only in the location of the third unsaturation, at either position 6 for gamma-linolenic acid (an omega-6 fatty acid) and at position 15 for alpha-linolenic acid (an omega-3 fatty acid). Here lipid-cholesterol interaction is used to distinguish the effect of po sition of unsaturation on membrane structure.