Preparation of biologically active platelet-derived growth factor isoforms AA and AB. Preferential formation of AB heterodimers.

We have expressed the mature platelet-derived growth factor (PDGF) A chain within a fusion protein of the cro repressor and beta-galactosidase in Escherichia coli. Monomeric PDGF-A was excised from this fusion protein by CNBr cleavage. After protection of thiols by S-sulfonation, this fragment was purified by gel permeation chromatography and reversed-phase high-performance liquid chromatography. The monomeric protein was dimerized in the presence of a mixture of reduced and oxidized glutathione to yield biologically active recombinant AA dimer (rPDGF-AA) with an overall yield of about 0.2 mg/l culture. When monomeric rPDGF-A and rPDGF-B were reacted at stoichiometric concentrations in the presence of glutathione, almost exclusively hetero-dimers of type AB were formed. Heterodimers AB stimulated [3H]thymidine incorporation into AKR-2B fibroblasts half-maximally at about 2 ng/ml. AA homodimers were fivefold less active. About 60,000 binding sites were found for rPDGF-AB, 30,000 for rPDGF-AA and 45,000 for rPDGF-BB on AKR-2B fibroblasts.