Composition of the neutral lipids of bovine meibomian secretions

Abstract

Lipids secreted from the bovine meibomian glands were assigned to the following classes: cholesteryl esters (A) (fatty acyl chain lengths from 15 to 27 carbon atoms), 41%; wax esters, 29%; triacylglycerols, 10%; and cholesteryl esters (B) (fatty acyl chain lengths from 14 to 18 carbons), 15%. The remaining 5% consisted of choles- terol, fatty acids, and highly polar material. Analysis of the lipids showed that only the cholesteryl esters (A) con- tained fatty acyl chains 2 Cz0:,. One-third of the fatty acyl chains of the wax esters and the cholesteryl esters (B) was iC15:o. The fatty alcohol moieties of the wax esters were found to be branched chain, Cz3:o-Cz7:o. A computer- based programmable gas-liquid chromatographic pro- cedure using 10% apolar 10C on Gas Chrom Q could be used to distinguish both is0 and anteis0 fatty alcohols and esters of fatty acids. The oily secretions of the meibomian glands are a major constituent of the precorneal tear film. These glands, located at the upper and lower margins of the eyelid, secrete materials that are thought to be responsible for the lubrication between the lid mar- gin and the cornea and that act as a hydrophobic barrier preventing evaporation of water from the tear film (1). The oils from the human meibomian glands are composed mainly of nonpolar lipids, principally cholesteryl esters and wax esters (2, 3), but they also appear to contain a surface active component (4). Analysis of the composition of the principal ester fraction from human meibomian secretions showed the major fatty acyl moieties to be nC18,1, UC~~:~, ic26:o and nC,,:,, while the major alcohols are cholesterol, ic26:0, nC2,:,, iC24:0, and nCZk0 (2). In this paper, we shall describe the characteriza- tion of bovine meibomian secretions. The principal nonpolar lipids, in particular the wax esters and cholesteryl esters, were separated by TLC and then the resulting fatty acyl and alcohol moieties were analyzed by GLC. MATERIALS AND METHODS