Evidence for a low-molecular-weight plasma peptide which stimulates chick chondrocyte metabolism.

Plasma contains a number of insulin-like activities (ILA) of molecular weights 7,000 to 90,000 (somatomedins and insulin-like proteins) which stimulate cellular metabolism and may function as growth factors. We have found evidence for the presence of an 800 Dalton peptide in human plasma which markedly stimulates the metabolism of chick chondrocytes. This peptide was extracted from human Cohn fraction IV-1 by procedures similar to those used for somatomedin isolations. At the Sephadex G-50 column separation step, the fraction with molecular weights of 300-1,000 was found to markedly stimulate chick chondrocyte metabolism. Rechromatography on Sephadex G-25 concentrated activity in peptides of molecular weight of about 800. An HPLC separation on a silica C-18 reverse phase column gave elution of the active peptide at 18% acetonitrile in water. This bioactivity appears to be a peptide which is free of lipids, carbohydrates, nucleic acids, metal ions, and immunoreactive insulin. This factor markedly increased the metabolism of cultured chick chondrocytes, but had only marginal activity on rat chondrocytes. When added at 1 microgram/ml to chick chondrocytes cultured in F-12 medium plus 1.5% fetal calf serum, the HPLC-purified activity increased DNA synthesis 7.3-fold, lipid synthesis 10.2-fold, and lactate production 2.9-fold after 48 h incubation. However, unlike somatomedins A and C, this factor did not displace insulin from placental membranes. These results suggest that low-molecular-weight peptides, which are smaller than the somatomedins, may contribute to the total ILA of human plasma.