The iodine-containing proteins of normal and abnormal human thyroid tissue

SOLUBLE FEATHER KERATIN 581 value.The analysis has indicated that histidine and lysine are absent from cysteic acid-keratin.The proportions of the other amino acids can be affected by the contaminants.The findings of Schroeder & Kay (1955) that the parts of feather differ slightly in composition can, of course, reflect a variation in the composition of the monomer, but are more likely to be due to different amounts of contaminants in the different parts.SUMMARY1.The reaction of soluble feather keratin with 1-fluoro-2:4-dinitrobenzene and with phenyl i8o- thiocyanate indicates that the protein contains only one-tenth equivalents of N-terminal amino acids/ mole of protein.2. The reaction of soluble feather keratin with carboxypeptidase and with the OH ion can be interpreted as evidence for the absence of a molar proportion of oc-carboxyl groups.3. Some of the amino acids in soluble feather keratin have been estimated quantitatively.4. It is concluded that the feather keratin monomer is an unbranched cyclic peptide.