Proteins of Human Hair and the Different Morphological Regions

Composition of the Proteins of Human Hair and the Different Morphological Regions 

A large number of investigations have been described regarding the analysis of the amino acids of whole human hair fibers. Whole-fiber amino acid analysis has several limitations because it provides average values for the amino acid contents of the average proteinaceous substances of the fibers. Therefore, for whole-fiber results, cross-sectional and axial differences in the composition of the fibers are averaged. 

A second complicating factor is hydrolytic decomposition of certain amino acids. The most commonly used medium for keratin fiber hydrolysis is 5 to 6 A hydrochloric acid. In studies involving acid hydrolysis of keratins, partial decomposition has been reported for cystine, threonine, tyrosine [9],phenylalanine, and arginine [10] with virtually complete destruction of tryptophan [11]. 

With these limitations in mind, the following discussion describes several important factors contributing to differences in the whole-fiber amino acid analysis results of human hair, reported in the literature. 

Unaltered human hair is hair that has not been chemically modified by treatment with bleaches, permanent waves, straighteners, or hair dyes. Numerous publications [6,12-27] describe results of the amino acid analysis of unaltered human hair. Table 2-1 depicts the structures for 21 amino acids that have been identified in human hair. Cysteic acid and other amino acids, derived from those amino acids of Table 2-1, are also present in either weathered or cosmetically altered hair, see Table 2-2. Table 2-3 summarizes results from several sources describing quantitative whole-fiber analyses of these 21 amino acids. These same amino acids are classified according to functional group in Table 2-4. 

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