Somatomedin-Like Effects of Biologically Active Bovine Growth Hormone Fragments*

Two fragments of bovine GH, obtained by dissociation of a limited tryptic digest of the hormone, were examined for several activities which have been associated with somatomedin. The activities evaluated were: 1) the in vitro uptake of [35S]sulfate and [3H]thymidine incorporation into DNA of cartilage segments of hypophysectomized rats and 2) the competition with [125I]iodo-insulin for insulin-binding sites of a particulate human placental membrane preparation. The smaller fragment, A-II, stimulated sulfation and DNA synthesis, and competed with insulin. The larger fragment, A-I, at a concentration of 1 × 10-7 M, enhanced DNA synthesis but did not affect sulfate uptake. A-I competed with insulin but not as effectively as A-II. Bovine GH, at the concentrations tested, did not affect sulfation or DNA synthesis. It did compete with insulin binding but not as effectively as A-II. The in vitro responses observed with both A-I and A-II were not parallel to those found with normal human serum, nor was the overall stimulation as great. The minimum effective concentrations of A-II for stimulating sulfation and DNA synthesis were 4 × 10-10 M and 4.5 × 10-8 M, respectively. These studies suggest that A-II (fragment 96–133 of bGH) possesses several activities (sulfation, DNA synthesis, and insulin competition) which are somatomedin-like. That A-I, like A-II, stimulated DNA synthesis and competed for insulin-binding sites is consistent with the suggestion that the GH molecule may have more than one “active site.”