PURIFICATION AND PARTIAL CHARACTERIZATION OF 2 TYPES OF GROWTH-INHIBITORY PROTEIN LATENTLY PRESENT IN RABBIT SERUM

Normal rabbit serum contained two kinds of growth-inhibitory protein, GI-I and GI-II, in latent forms. These latent inhibitors were activated by incubation at 37-degrees-C for 12 h, and their activation was lowered by inhibitors for serine, cysteine and metalloproteinases. Both growth inhibitors were highly purified in active forms by successive column chromatographies. GI-I showed a major protein band with an M(r) of 18 000 on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, while GI-II showed a major protein band with an M(r) of 36 000. GI-I and GI-II half-inhibited the growth of rat tumorigenic cell line (RSV-BRL) at concentrations of 0.5 ng/ ml and 10 ng/ ml, respectively. Their effects were cytostatic and reversible at low concentrations, but cytotoxic and irreversible at excess concentrations. Of the 15 cell lines tested, GI-I specifically inhibited the growth of rodent and lagomorph cells, whereas GI-II nonspecifically inhibited the growth of all cell lines tested. Specificities for cell type and malignancy were not observed with either inhibitor. These growth inhibitors were stable to a reducing reagent and proteinase inhibitors, but labile to urea, acid organic solvents, trypsin, plasmin and heating at 95-degrees-C for 5 min. These properties suggested that both growth inhibitors might be distinct from known growth-inhibitory factors.