18.    Origins and importance of 18 methyl eicosanoic acid (18MEA) in human hair.
Louise Albiston(2), Maf Davis(1), Preyesh Parmar(1), Godfrey Harrap(2), Steven Goff(1), Gill Westgate(3)   1.Unilever R&D Colworth, Sharnbrook, Bedford,. 2 Unilever R&D Port Sunlight, Bebington, Wirral, 3 Westgate Consultancy, Stevington, Bedfordshire, UK.

Hair fibres contain complex lipid membranes which are likely to confer important properties to the hair fibre but whose organisation and composition is poorly understood. The lipids in the hair fibre originate either from the follicle or are sequestrated from sebum. Differential extraction can be used to resolve the ‘free’ and ‘bound’ lipid fractions. Hair contains an unusual branched chain fatty acid, 18MEA, believed to be covalently bound within the cuticle. 18MEA has been described by a number of groups, in particular in relation to absence in the hair from subjects with maple syrup urine disease (MSUD), a defect in the metabolism of branched chain amino acids. In order to study the biosynthesis of integral hair lipids including 18MEA, human hair follicles were isolated from facelift scalp skin and metabolically labelled using 14C-acetate and 14C-isoleucine. The hair fibres grown in vitro were isolated; the covalently bound fatty acids extracted and separated using reversed phase HPLC. Using appropriate standards a biosynthetic profile of hair follicle derived integral fatty acids was determined. The same extraction and analysis methods were used to characterise integral fatty acids in a range of hair samples, including fibres from subjects with MSUD. Furthermore, hair samples from MSUD patients and age/sex/ethnic matched samples were examined by TEM to re-examine any cuticular abnormalities associated with MSUD.
Results show that 18MEA is predominantly synthesised from isoleucine, demonstrating the pathway for conferring the c21 methyl branch to the molecule. Hair samples from subjects with MSUD showed a deficiency in 18MEA, but morphologically and ultrastructurally the hair fibres appeared normal.
In summary, hair fibres are variable with respect to their overall integral lipid composition which may explain some of the differences in hair fibre properties between individuals. The MSUD results suggest that 18MEA, although unusual, confers no unique ultrastructural feature to hair fibres.