Purification from Bovine Serum of a Survival-Promoting Factor for Cultured Central Neurons and Its Identification as Selenoprotein-P

We purified from bovine serum a glycoprotein that promotes the survival of rat embryonic neurons cultured from septum and other brain regions. A 40,000-fold purification was achieved by using a combination of ammonium sulfate precipitation, Zn2+ affinity chromatography, Cibacron blue 3-GA dye affinity chromatography, ABx ion exchange chromatography, and preparative PAGE. The active protein had an apparent molecular weight of 50-60 kDa. The concentration required for half-maximal survival (EC50) was 12 ng/ml ( approximately 200 pM) for the final fraction. Amino acid sequencing after cyanogen bromide cleavage yielded two sequences that are homologous to regions of deduced sequence of the selenoprotein-P (SPP) family in bovine, rat, and human. Antibodies against a synthetic peptide within the bovine SPP sequence immunoprecipitated and inhibited the survival-promoting activity of a partially purified serum fraction. The purified protein supported neuronal survival more effectively than inorganic selenium. These results suggest that SPP or an SPP-like protein contributes to the neuronal survival-promoting activity of serum.