Isolation and Amino‐Acid Sequence of the C‐Peptide of Duck Proinsulin

C‐peptide from the duck was isolated and its amino acid sequence determined. The duck C‐peptide contains 26 amino acids and differs substantially from the known mammalian C‐peptides. It is the first C‐peptide shown to contain the amino acids histidine, tyrosine and phenylalanine. The sequence was found to be as follows: Asp‐Val‐Glu‐Gln‐Pro‐Leu‐Val‐Asn‐Gly‐Pro‐Leu‐His‐Gly‐Glu‐Val‐Gly‐Glu‐Leu‐Pro‐Phe‐Gln‐His‐Glu‐Glu‐Tyr‐Gln. A comparison of duck C‐peptide with the known mammalian C‐peptides shows that the terminals are homologous and that the five invariable positions of the nine known mammalian C‐peptides can be retained by means of four deletions in the duck C‐peptide. It is likely that their common ancestor, the reptile C‐peptide, contained at least 31 amino acids and that deletions at various positions were involved in the evolution of ox, pig, dog and duck C‐peptides