Isolation of a Somatomedin From Plasma of Rats Bearing Growth Hormone Secreting Tumors*

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Abstract
A protein which has somatomedin-like properties was purified 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 MW range with a minor proportion in the 50,000 MW 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 MW range. This material had an isoelectric focusing range of 8.4-9.6, similar to that described for human somatomedin C. On SDS[sodium dodecyl sulfate-1]-urea polyacrylamide gel electrophoresis only 1 protein band was seen. The isolated peptide (rSm) stimulated sulfate uptake in hypophysectomized rat cartilage. The potency of 2 preparations was variously assayed from 14.0-54.7 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 porcine 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 > normal rat sera > hypophysectomized rat sera.