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Amniotic fluid contains materials other than insulin which react in a somatomedin C radioreceptor assay using human placental membranes. The material in mid-gestational amniotic fluid which reacted with the somatomedin C radioreceptor assay eluted slightly after albumin from a Sephadex G-150 column equilibrated with 0.1M NH4HCO3. Neither boiling nor treatment of this fraction with 1% formic acid yielded small molecular weight somatomedin-like peptides. Separation of the somatomedin reactive material (Sm RM) from albumin was achieved by gel filtration through Ultrogel AcA44 in 3.1M NH4HCO3. The active product had an apparent molecular weight of 33,000 to 35,000 Daltons; its isoelectric point determined by focusing was between 4.1 and 5.1. The purified amniotic fluid protein displaced somatomedin C in the somatomedin C radioreceptor assay but did not compete with insulin in the insulin radioreceptor assay. Sm RM produced only a slight stimulation of thymidine uptake in human fibroblasts but was inactive in stimulating sulfate uptake in hypox rat costal cartilage. In human fibroblast cultures Sm RM inhibited the stimulation of thymidine uptake induced by human serum and by purified rat somatomedin. When Sm RM was added to the 125I somatomedin C, some of the radioactivity eluted from gel filtration at pH 8.6 was converted to a molecular weight complex of about 43,000. We conclude that the material which we have isolated from mid-gestational amniotic fluid is a protein which may bind somatomedin and make it unavailable to the somatomedin receptor of human placenta and human fibroblasts.

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A B S T R A C T Recently, evidence has been reported to suggest that human platelets like several other circulating blood cells may bind insulin. To examine whether human platelets contain specific insulin receptors, washed human platelets suspended in Hepes buffer were incubated at 24°C with 1251-insulin in the presence and absence of unlabeled insulin and specific insulin binding was determined. Insulin binding by platelets increased progressively with time of incubation to reach a maximum at 3 h and was proportional to the number of platelets in the incubation mixture. Maximum insulin binding was observed at pH 8. Insulin degradation by platelets as assessed by TCA precipitability and reincubation studies was minimal. Scatchard analysis ofthe binding data and dissociation studies revealed evidence of negative cooperativity of the platelet insulin receptor. A high affinity dissociation constant of _3 x 109 M-1 was determined and the concentration of platelet insulin receptors was estimated as 25 binding sites/,un2 platelet surface area. Binding of 1251-insulin by platelets was inhibited by unlabeled porcine insulin and to a lesser extent by catfish insulin and porcine proinsulin but not by glucagon, prolactin, growth hormone, and thrombin. The findings indicate that human platelets contain specific insulin receptors. The significance of the platelet insulin receptor, particularly with respect to altered platelet function in diabetes mellitus, remains to be determined.

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Fifty pglml ascorbic acid was found to accelerate the growth of logarithmically growing chick embryo sternal chondrocytes in cell culture. Differences in the number of cells between control and ascorbate-treated cultures were apparent by the fourth day of treatment when a 58% increase in the number of cells and a 35% increase in protein per culture were observed in ascorbate-treated cultures. Under these conditions, the cell size and protein content per cell was reduced in ascorbate-treated cultures.Collagen synthesis, which was measured by the amount of collagenase-sensitive counts released from 3H-proline labeled protein, was increased by 116% over controls after 24 hours of ascorbate treatment. Observations with scanning and transmission electron microscopy are consistent with the idea that ascorbate has a much greater effect on collagen secretion and demonstrate an ascorbate-mediated change in matrix morphology.35S incorporation into hyaluronidase-sensitive or cetylpyridinium chloride precipitates was accelerated by 57% by 24 hours of ascorbate treatment whereas two, four and six days of treatment caused decreases in this parameter of 19%, 57% and 3596, respectively. This effect was dependent on cell density. 35S incorporation probably reflects synthesis of new glycosaminoglycan because similar results were observed when either 35S or 14C-glucose incorporation was monitored.An effect of ascorbate on both collagen and glycosaminoglycan synthesis is consistent with the possibility that collagen and glycosaminoglycan synthesis can be coregulated under some circumstances.The effect of ascorbate on collagen accumulation is well known. Deficiency of this vitamin results in scurvy, a clinical syndrome characterized by tissue fragility, hemorrhage, impaired wound healing and anemia (Chazan and Mistilis, '63). Cartilage is affected by scurvy. Effects are seen in epiphyseal cartilages of long bones in the guinea pig where there is reduced matrix (Wolbach and Maddock, '52). In addition, in scurvy sulfate incorporation is reduced to one third of normal levels (Reddi and Norstrom, '54).The vitamin is a cofactor for both prolyl and lysyl hydroxylases (Peterkofsky and Udenfriend, '53; Hausman, '67), and 3T3 fibroblasts and chick embryo fibroblasts exhibit a dependence on ascorbate for maximal proline hydroxylation and for optimal collagen secretion (Peterkofsky, '72a). Similarly in chick embryo chondrocytes, Lavietes ('71) has reported increased hydroxylation of proline in the presence of ascorbic acid. Chen and Postlethwait ('70), on the other hand, found that ascorbate did not affect total proline incorporation into the collagen of sponge induced connective tissue growth in vitro but increased the appearance of hydroxyproline, suggesting that the effect of the vitamin in their system was not on collagen synthesis but rather only on proline hydroxylation. Similarly, Peterkofsky (72b) found that the rate of collagen synthesis was not affected by ascorbate in L-929 cells in culture when using an assay which '

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