Enhancement of the Hyperglycemic Activity of Human Growth Hormone by Enzymic Modification

Abstract

Limited hydrolysis of human growth hormone [GH] with subtilisin (Novo) converted the hormone to a mixture of 3 modified forms: S1, S2, S3. These were 2-chain modifications formed by cleavage of the peptide chain within the large disulfide loop of the hormone in the region between residues 137-149. Each of these substances was administered to dogs as a single s.c. injection 10 h prior to an oral glucose tolerance test. The S1 form was the most active in causing hyperglycemia and hyperinsulinemia. The S2 and S3 forms also produced diabetogenic responses but the magnitude of those effects was less than with the S1 form. GH before enzymic hydrolysis, tested at 10 times the dose used for S1, produced no abnormality in either the glucose tolerance curves or in the insulin levels. All 3 modified forms had excellent growth promoting activities when tested by the tibial line assay. Attempts were made to isolate the naturally occurring pituitary substance that produces glucose intolerance in dogs. An active fraction could be separated from preparations of clinical grade GH. The GH remaining after removal of the active material caused only slight hyperglycemia. Intact human GH was only weakly hyperglycemic but when converted to a 2-chain structure by subtilisin, the hormone became extremely active in producing glucose intolerance and hyperinsulinemia. The relationship of this subtilisin-cleaved material to the naturally occurring diabetogenic substance of the pituitary gland must await purification of the natural form.