Isolation of a somatomedin from plasma of rats bearing growth hormone secreting tumors.

We have purified a protein which has somatomedin-like properties from the serum of Wistar-Furth rats bearing a growth hormone producing pituitary tumor (MStT/W15). Activity was measured by a placental insulin and/or somatomedin C radioreceptor assay (SmC-RRA). The serum was initially filtered through Sephadex G-150 equilibrated with 0.1 M NH4HCO3 and 0.02% NaN3. On the G-150 column, radioreceptor insulin (RRI) and radioreceptor somatomedin C (RRSm-C) activities coincided and appeared predominantly in the 160,000 mol wt range with a minor proportion in the 50,000 mol wt range. The pooled active fractions were boiled for 30 min at pH 5.5. After removing denatured protein by centrifugation, the extract was passed through G-50 Sephadex equilibrated with 1% formic acid and 0.15 M NaCl. Sixty to 90% of the SmC-RRA activity in the effluent appeared in the 9000 mol wt range. This material has an isoelectric focusing range of 8.4--9.6, similar to that described for human somatomedin C. On SDS-urea polyacrylamide gel electrophoresis only one protein band was seen. The isolated peptide (rSm) stimulated sulfate uptake in hypophysectomized rat cartilage. The potency of two preparations was variously assayed from 14.0 to 54.7 units/mg. Rat somatomedin was iodinated and purified by absorption on and elution from placental membranes. Eight to 12% of rat [125I]Sm was specifically bound by human placental membranes. Rat [125I]Sm was displaced by hSmC and rSm and human NSILA-S, partially displaced by procine proinsulin and poorly displaced by rat insulin. In preliminary studies, rat [125I]Sm was displaced from receptors on human placental membranes by sera from pituitary tumor bearing rats greater than normal rat sera greater than hypophysectomized rat sera.