Complementation of human growth hormone (GH) peptide 1-134 with C-terminal fragments of human GH produced by digestion with bromelain

The digestion of human GH (hGH) with the proteolytic enzyme, bromelain, results in a major product consisting of a mixture of three large fragments, i.e. residues 1-135 + 143-191, 1-135 + 145-191, and 1-135 + 146-191. In the case of each fragment, the N-terminal peptide is joined to the C-terminal fragment by the disulfide bridge between residues 56 and 165. A C-terminal fragment mixture consisting of peptides 143-191, 145-191, and 146-191 was isolated from this major digestion product after reduction and S-carbamidomethylation of its disulfide bonds. In the present study, the noncovalent complementation of the peptides in this mixture with S-carbamidomethylated peptide 1-134 derived from thrombin-digested hGH was investigated. Noncovalent complementation of these peptides was accomplished by dissolving equimolar amounts of the materials in 0.5% ammonium bicarbonate-6 M guanidine-HCl and dialyzing the mixture slowly to remove the guanidine-HCl. The recombinant mixture was recovered in 26% yield by gel filtration of the peptide mixture and was found to contain three noncovalent recombinant species, i.e. peptides 1-134 + 143-191, 1-134 + 145-191, and 1-134 + 146-191. Thus it would appear that residues 135-145 are not required to obtain noncovalent complementation between the N- and C-terminal regions of the hGH molecule. In an RIA for hGH the recombinant mixture was found to possess approximately 40% the cross-reactivity of the native hormone. In contrast, it had only about 10% the activity of native hGH in the weight gain test in hypophysectomized rats, in stimulating phenylalanine incorporation into the protein of the isolated hypophysectomized rat diaphragm, and in stimulating glucose oxidation by isolated adipose tissue of hypophysectomized rats. The limited biological activity of the recombinant mixture is of interest, since the major bromelain digestion product from which the C-terminal peptides were derived consists of a mixture of rather similar molecules (i.e. peptides 1-135 + 143-191, 1-135 + 145-191, 1-135 + 146-191; and with intact disulfide bridges), which exhibits substantial growth-promoting and insulin-like activities.