Cuticle or cortical cell surface in hair are abundant in fatty acids
unlike the keratinized area of epidermis or sebaceous gland in hair and about 30–40% of such fatty acids were composed of 18-
methyl-eicosanoic acid (18-MEA) and known to be bound to
proteins by ester or thioester bond in living or keratinized cells
[8,11]. Alkaline treatments (hair colorings, bleaching, perms) or
sunlight can lead to a loss of 18-MEA and causes the defatting of
hair. Consequently hair becomes dry and brittle and loses its
springiness and luster [24]. The synthesis of 18-MEA requires the
oxidative decarboxylation of isoleucine. Maple syrup urine disease
(MSUD) is caused by an inherited deficiency in the enzyme
branched chain 2-oxo acid dehydrogenase, which leads to the
accumulation of branched chain keto-acids derived from the
amino acids, leucine, isoleucine, and valine. Transmission electron
microscopy studies of developing hair fibers show a structural
defect in the fiber shaft in hair from patients with MSUD. This
defect is confined to the cuticle of the fiber, where the cuticle membrane directly apposes to the intercellular material. The
defect indicates that 18-MEA is located exclusively on the upper
surface of fiber cuticle cells [25].