Polypeptides or proteins are produced as a disulfide bridged dichain by recombinant expression in E. coli host cells and exert biologic activity as such a dichain. A C-terminal amino acid of the first chain is Arg or Lys. The second chain has N-terminally 1 to 20 amino acid residues and a PRS sequence VPXGS, wherein X is a natural amino acid； V is Val, Leu, Ile, Ala, Phe, Pro or Gly； P is Pro, Leu, Ile, Ala, Phe, Val or Gly； G is Gly, Leu, Ile, Ala, Pro, Phe or Val； S is Ser, Tyr, Trp, or Thr. The polypeptide/protein is modified at the nucleic acid level to a construct that in a loop area has a PRS sequence VPXGS with X, V, P, G, and S as defined. The construct is inserted into E. coli host cells that are cultivated and subsequently lysed for isolating the dichain disulfide-bridged peptide/protein.
