1. The polypedtide chains that comprise the subunits of the tonofilaments, or the α-keratin component, of bovine epidermis were fractionated by combination of chromatography on DEAE-cellulose and preparative polyacrylamide-gel electrophoresis. 2. The seve polypeptide chains investigated had generalyy similar properties; all contained two residues per molecule of tryptophan and N-acetylserine was the common N-terminal amino acid residue. 3. On the basis of close similarities in α-helix content and amino acid composition, the polypeptide chains were classified into three distinct groups. Each group contained approximately one-third of the total polypeptides on a molar basis. The groups and designated polypeptides chain numbers were: group one, polypeptides 1a and 1b, which had moleculae weights of 58,000, contained about 25% α-helix, 86 glutamic acid and 8 cysteine residues per molecule, but which differed in net charge, extinction coefficients and tyrosine contents; group two, polypeptides 2, 3, and 4, which hadmolecular weights within thewithin the range of 52,00-56,000, contained about 48% α-helix, 54 glutamic acid and 6 cysteine residues per molecule, but which differed in extinction coefficients and tryosine contents; and group, polypeptides 5 and 6, which had molecular weights of 47000-48000, contained about 56% α-helix, 64 glutamic acid and 4 cysteine residues per molecule, but which differed in extinction coefficients and tyrosine contents, it is suggested that none of the chains is a precursor or a degradation product of other polypeptidc chains. 5. It is concluded that bovine epidermal α-keratin consists of a heterogeneous group of similar polypeptide chains.

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