Endocytosis of transcobalamin in male rabbit germ cells: electron microscope radioautography study

The binding of 125I-iodinated transcobalamin to a suspension of isolated rabbit germ cells was studied by Scatchard plot. The number of binding sites was evaluated to about 1000 per cell, and its association constant (Kass) in order of 14.6 l/nmole. The distribution to structures related to endocytosis was determined by ultrastructural histomorphometric studies. Both coated and uncoated structures were present regardless of maturation stage. The number of coated vesicles was at its highest in the initial maturation steps, whereas the number of uncoated vesicles was highest in the final maturation steps. The endocytosis of 125I-iodinated transcobalamin by the suspension of germ cells was studied by electron microscope radioautography. The tracer was mostly detected over the plasma membrane, coated vesicles and multivesicular bodies of germ cells. The grains were observed mainly over spermatocytes and round spermatids; 31.6% and 32% of these cells, respectively, were labeled. In contrast the tracer was detected in only 8.11% of elongated spermatids. In conclusion, iodinated transcobalamin is internalized in rabbit germ cells by receptor-mediated endocytosis. This phenomenon was predominant in the early stages of germ cell maturation.