Transferrin receptors of isolated rat seminiferous tubules bind both rat and human transferrin.

The binding and uptake of rat and human transferrin by isolated rat seminiferous tubules was studied. During the isolation and incubation of the tubules, the blood-testis barrier remained intact. Iron-saturated and iron-free (apo-) transferrin use the same binding sites on the surface of the tubules, but the dissociation constant is about two times higher for apotransferrin than for iron-saturated transferrin. The affinity of the receptors is equal for rat and human transferrin, but human transferrin binds to more surface binding sites (2.6 X 10(10) per 10 cm tubule length) than rat transferrin (1.1 X 10(10) per 10 cm tubule length) at 0 degrees C. At 33 degrees C equal numbers of human and rat transferrin molecules are taken up (about 8 X 10(10)) per 10 cm tubule length. The quantitative difference between 0 degrees C and 33 degrees C is caused by the fact that at 33 degrees C receptor-mediated endocytosis and recycling occur. As a consequence, both surface and intracellular transferrin receptors are detected at 33 degrees C. The dissociation constants are not temperature-dependent.