Subunit composition, organic anion binding, catalytic and immunological properties of ligandin from rat testis.

Rat testicular and liver cystols contain ligandin as determined immunologically, and have high glutathione-S-transferase activity. Unlike liver cytosol, testicular cytosol does not contain protein components that bind bilirubin or sulfobromophthalein with high affinity. To investigate these effects, ligandin was purified to homogeneity from rat testis. Whereas rat liver ligandin consists of equal amounts of two subunits with molecular weights of 22,000 (Ya) and 25,000 (Yb), more than 90% of testicular ligandin consists of Yb. Rat testicular ligandin is immunologically similar to liver ligandin, and has identical glutathione-S-transferase activity, but lacks the capacity for high affinity binding of bilirubin and sulfobromophthalein. The amino acid composition and other properties of testicular ligandin are similar to those of the Yb subunit of liver ligandin. Sulfobromophthalein and bilirubin biphasically inhibit the glutathione-S-transferase activity of liver ligandin: initial high affinity inhibition is followed by reduced inhibition. Testicular ligandin has only low affinity inhibition kinetics. These results suggest that Ya is required for high affinity binding, and that reduced organic anion binding by testicular ligandin results from the lower amounts of Ya in testis.