HUMAN HAIR KERATINS

Human hair keratins were among the first to be studied but it is only recently that sufficient information has been obtained to gain a basic biologic perspective of these proteins. Hair keratins are members of the intermediate filament family of proteins, yet are sufficiently div ergent from epidermal keratins to warrant separate classification: typ e Ia and IIa (''hard''/hair keratins) and type Ib and IIb (epidermal a nd other ''soft'' keratins). As with hair keratins from other species, the human proteins may be distinguished from their epidermal counterp arts by a relatively higher cysteine content, 7.6% versus 2.9%, respec tively. This feature reflects utilization of disulfide bonding in prod ucing a tougher, more durable structure in the tissues in which the ha ir keratins are distributed. Although prominent in hair, their distrib ution is not strictly limited to this tissue. A number of molecular ch aracteristics have been elucidated from human hair keratin gene studie s including amino acid sequence data for a type la hair keratin. Studi es of various pedigrees has revealed a fairly wide latitude of variati on in human hair keratin expression that is tolerated without associat ed obvious hair phenotypic change. Thus, a foundation of knowledge reg arding these proteins has emerged and continues to evolve.