Binding of metallothionein to rat spermatozoa

Binding studies of metallothionein and rat spermatozoa were performed using 125I-Tyr-metallothionein (125I-MT). Reactions between 125I-MT and spermatozoa indicated that MT bound in two forms, namely, middle affinity (Kd1 = 2 x 10(-9) M) binding and low affinity (Kd2 = 1 x 10(-8) M) binding. Labeled MT binding to spermatozoa was inhibited by adding anti-MT antibody. Total binding reactions of MT were temperature and incubation time dependent. By transmission electron microscopy using a gold conjugate (indirect method), gold particles bound to MT were shown to bind to the cell membrane of the head and the proximal portion of the tail. By optical autoradiography, grains of labeled MT were localized mainly in the head and the proximal portion of the tail. By electron microscopical autoradiography, grains of labeled MT were identified mainly in the cell membrane of the head and tail and partly in the nucleus. These results suggest that MT has both specific and non-specific binding sites on the spermatozoal membrane.