Platelet-derived growth factor. Isolation by a large-scale procedure and analysis of subunit composition.

Platelet-derived growth factor was purified from fresh platelets by a large-scale procedure not involving the use of SDS (sodium dodecyl sulphate). The product, 0.5 mg of platelet-derived growth factor, obtained from about 3 x 10(13) platelets migrated as a single component in analytical gel electrophoresis in the presence of SDS and showed no inhomogeneity on sedimentation-equilibrium analysis in the ultracentrifuge. It had a high specific activity, 2 ng of platelet-derived growth factor/ml (70pM) being equivalent to 1% (v/v) human serum in an assay for multiplication-stimulating activity. Amino acid analysis revealed that platelet-derived growth factor contains all the common amino acids, except tryptophan, but no hexosamine. The molecular weight of platelet-derived growth factor, as determined by sedimentation-equilibrium analysis, was about 33 000. A similar value was obtained by gel electrophoresis in SDS under non-reducing conditions. In the presence of reducing agents the factor molecule was converted into two distinct components of lower molecular weight (17 000 and 14 000 respectively), as demonstrated by protein staining. The molecular model implicated by these findings is that platelet-derived growth factor consists of two different polypeptides chains, linked by disulphide bridges.