Comparative biochemical analysis of purified S-antigen from human, bovine, porcine and rat retina.

Highly purified S-antigen from human, bovine, porcine and rat retina was used to ascertain the molecular weights, isoelectric points, amino-acid composition and presence of carbohydrate moieties. All S-antigen preparations comigrate to 50,000 MW on SDS-polyacrylamide gels and demonstrate microheterogeneity upon isoelectric focusing. Human S-antigen exhibits two bands at pH 5.9 and 5.6, whilst with bovine S-antigen a broad band at pH 5.9 was observed. Porcine and rat S-antigen gave four bands, focusing between pH 5.0-5.6. Two-dimensional gel analysis revealed that all the four focused porcine polypeptides migrate to 50,000 MW. Western-blot analysis following isoelectric focusing of human and porcine S-antigen showed that all polypeptide bands react specifically with polyclonal and monoclonal antibodies to S-antigen. The reasons for the apparent heterogeneity of S-antigen by isoelectric focusing are discussed. All S-antigen preparations were weakly positive for periodic acid Schiff staining, and specifically bound radiolabelled Lens culinaris lectin on Western-blot analysis. These results confirm the glycoprotein nature of S-antigen which may explain the finding of multiple bands found on isoelectric focusing. Amino-acid analysis reveals the high percentage of non-polar amino acids and may explain the apparent hydrophobic behaviour of the isolated protein.