Our previous studies provided evidence that guinea pigs that are vitamin C deficient and those that have been fasted, but supplemented with vitamin C, are equivalent with respect to the mechanisms responsible for decreased collagen and proteoglycan synthesis. Sera from both groups contain a circulating factor that inhibited DNA synthesis in 3T3 cells and collagen and proteoglycan synthesis in cultured primary chondrocytes and adult, but not fetal, human' skin fibroblasts. The presence of the inhibitor in fasted and scorbutic sera was correlated with an increase in low molecular weight insulin-like growth factor (IGF)-binding proteins (30-50 kDa) that can inhibit binding of IGF-I to its receptor on 3T3 cells and human fibroblasts. Our recent studies are concerned with identifying the IGF binding proteins in guinea pig sera. One of the IGFBPs that is increased in scorbutic and fasted guinea pig serum has been identified as IGFBP-l, a 30-kDa protein, by using ligand blotting, Western blotting and immunoprecipitation. The second binding protein that is increased has been tentatively identified as IGFBP-2 based on its ligand binding specificity and size (35 kDa) observed on ligand blots. IGFBP-3 could not be observed by the previous methods that depended on the presence of a free IGF binding site, since it exists in serum as a complex with the IGFs. It can be detected on ligand blots since complexes are dissociated during SDS-PAGE. We identified a doublet of 41 and 44-kDa binding proteins as glycosylated forms of BP-3 and their levels were unchanged during scurvy and fasting. Characterization of the induced proteins and studies to confirm their function as inhibitors of IGF-I function are continuing.
