Quantitation and characterization of a newly described organic anion binder by radioimmunoassay

We have recently identified a new group of proteins in rat liver cytosol which possess binding properties comparable to those of the glutathione-S-transferases, yet lack transferase activity. The bulk of organic anion binding in this fraction was associated with the tetrameric protein, designated the organic anion binder, molecular weight 34 000 (Sugiyama, Y., Yamada, T. and Kaplowitz, N. (1982) Biochim. Biophys. Acta 709, 342-352). In order to determine its tissue distribution and molecular forms, a sensitive and specific radioimmunoassay was developed. Gel filtration of rat liver and kidney cytosol demonstrated only one peak of organic anion binder-like immunoreactivity corresponding to a molecular weight of 35 000. Organic anion binder-like immunoreactivity was present in 14 separate tissues. The greatest concentration of the organic anion binder was found in the liver and kidney cytosol, where it constituted 0.5% of the cytoplasmic proteins. Similar hepatic contents were noted for female and male rats. The content of the organic anion binder in the liver and kidney were low in the fetus, but approached adult levels by 2-4 weeks of age. Treatments known to augment hepatic glutathione-S-transferase content had minimal effect on organic anion binder content. In summary, the organic anion binder is a newly recognized cytosolic binding protein, distinct from the glutathione-S-transferases, present in greatest concentration in the liver and kidney. The combination of the protein's binding properties along with its location and ontogeny suggests its possible role in intracellular organic anion transport.