Trypsin-resistant forms of human growth hormone have diabetogenic and insulin-like activities

Although diabetogenic and insulin-like activities are intrinsic properties of the growth hormone (GH) molecule, it has been frequently suggested that the hormone must be proteolytically processed for these activities to be expressed. If this is correct, then derivatives of GH having resistance to appropriate proteolytic attack might not have diabetogenic and/or insulin-like activity. The purpose of the present study was to prepare derivatives of human GH that are resistant to digestion by trypsin and to determine whether they possess diabetogenic or insulin-like activity. Three derivatives were prepared from purified native human GH in which lysine residues were modified with methyl acetimidate, citraconic anhydride or S-ethyl-thioltrifluoroacetate, and one in which arginine residues were modified with camphorquinone-10-sulfonic acid. Comparisons of peptide maps of tryptic digests of these derivatives with that of unmodified human GH indicated that all four were resistant to proteolysis by trypsin. All of these trypsin-resistant forms of human GH were found to possess significant growth-promoting, diabetogenic and insulin-like activities, although all activities were attenuated to some extent in each derivative. The relative potencies of the human GH derivatives in a radioimmunoassay for human GH were somewhat similar to their order of potency in the growth-promoting and diabetogenic assays. These results suggest that if proteolytic processing of the GH molecule is involved in the expression of one or more of its biological activities, such processing probably does not involve a trypsin-like proteinase.