An alpha-one protein is separated from human serum on a microbead column. This nondialyzable protein induces the immediate growth of unadapted cells placed in chemically defined Medium A2 + APG. HeLa, conjunctiva and human heart cells, which stop growing if the protein is removed, continue to grow only if the protein is returned or the cells are permitted to adapt to the defined medium. A 90–120 day period is required for adaptation. The spreading and growth of fully adapted cells is also stimulated by the addition of the protein. As little as 0.4 µg per ml of medium is effective. The protein analyzed by paper, starch, and discontinuous acrylamide gel electrophoresis appears to be a single component. The protein is periodate-Schiff positive and readily binds small molecules which are removed, without loss of biological activity, by precipitating the protein in 50% saturated ammonium sulfate. The protein is adsorbed on the microbead column as a complex with the beta lipoprotein fraction of human serum; it cannot be separated from bovine or equinesera by this method; and it is not identical with fetuin. Its biological response is not duplicated by insulin, carbamyl phosphate, putrescine, or linoleic acid.

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