Iodination of Rabbit Sperm Plasma Membrane: Relationship of Specific Surface Proteins to Epididymal Function and Sperm Capacitation1

To evaluate the changes which mammalian sperm membranes undergo during maturation and fertilization, methodology was developed to incorporate I125 into the surface components of epididymal and ejaculated spermatozoa. Optimal conditions for iodination were obtained at 10−4 M KI, 5 × 10−5 M H2O2 and 0.15 mg/ml lactoperoxidase. Under these conditions ∼ 106 atoms of iodine were bound/ sperm. Treatment with trypsin resulted in removal of 50–75% of the bound counts in one h. Conditions of iodination were sufficiently gentle so as not to cause any statistical change in motility between iodinated and uniodinated sperm samples and no loss in the ability of iodinated spermatozoa to fertilize ova in vivo. When samples of iodinated epididymal or ejaculated sperm were subjected to SDS electrophoresis, each showed up to 7 iodinated components, 5 of which were similar for both epididymal and ejaculated sperm. Of the 2 remaining components, one was unique to ejaculated sperm while the other was associated with sperm obtained from the epididymis. When the sperm were subjected to a capacitating environment, one major component was eluted from the surface of the sperm. This component has a relative mobility on electrophoresis close to that previously reported for the acrosome stabilizing factor.