CHARACTERIZATION OF DOPUIN, A POLYPEPTIDE WITH SPECIAL RESIDUE DISTRIBUTIONS

A 62-residue polypeptide, dopuin, has been isolated from pig small int estine. It is distinguished by an N-terminal part with a high content of proline (7 in a 26-residue segment), a C-terminal part with a high proportion of histidine (3 in a 9-residue segment), and six half-cysti ne residues in three intrachain disulphide bridges (connecting positio ns 22-25, 23-54 and 35-44). The Cys and Pro distributions suggest a ti ght and special conformation. In contrast to PEC-60 and somatostatin, it has no established inhibitory effect on insulin secretion. At 10 nM concentration, a weak inhibitory tendency is less than half of that o f the other two peptides. Like gastrointestinal trefoil peptides. dopu in has three disulphide bridges, Ala-Pro segments, and many charged re sidues, but they are differently distributed and dopuin belongs to a s eparate, apparently novel family. However, dopuin is similar to a pept ide corresponding to an expressed sequence-tag cDNA of human fetal liv er and spleen, establishing the nature of the mature form of tile prod uct of this cDNA, and showing a general tissue, age, and species distr ibution of this peptide. A truncated form of vimentin, composed of its C-terminal 37 residues, vimentin-C37, was also purified and structura lly characterized. These two peptides increase the complexity of known intestinal polypeptides and at least dopuin has properties compatible with specific biofunctions.