Human platelet-derived growth factor. Purification and resolution into two active protein fractions

Human platelets secrete a factor that stimulates cultured human cells to initiate DNA synthesis and to divide. This human platelet-derived growth factor (PDGF) has been purified ≃100,000-fold into two equally active homogeneous fractions, PDGF I (M[r] ≃31,000) and PDGF II (M[r] ≃28,000). The amino acid compositions of each are similar, highly basic, and show 18 half-cystine residues. Both PDGF I and II are glycoproteins, but differ in their carbohydrate compositions. The data suggest that PDGF II may be a proteolytic cleavage product of PDGF I but do not rule out that the proteins may be separate but very similar gene products. Purified PDGF is active in stimulating DNA synthesis at 0.2 ng/ml.