The present invention relates to a vector for expression of a heterologous protein by a Gram negative bacteria, wherein the vector includes a nucleic acid such as DNA encoding the following: an origin of replication region; optionally and preferably a selection marker; a promoter; an initiation region such as translation initiation region and/or a ribosome binding site, at least one restriction site for insertion of heterologous nucleic acid, e.g. DNA, encoding the heterologous protein, and a transcription terminator. The inventive vector may contain DNA encoding the heterologous protein, e.g., pro-insulin such as pro-insulin with a His tag. Additionally, the invention provides a method for extracting a recombinant protein from within a recombinant Gram negative bacteria having a cell membrane, without lysing the bacteria, as well as a method for purifying an isolated recombinant human insulin, wherein the isolated recombinant human pro-insulin is subjected to sulfitolysis, Ni-chelation chromatography, renaturation, limited proteolysis and chromatography separation to provide purified, isolated, recombinant human insulin.