Biosynthesis of rat sperm outer dense fibers during spermiogenesis. In vivo incorporation of [3H]leucine into the fibrillar complex.

The temporal incorporation profile of [3H]leucine into the outer dense fiber polypeptides was determined after the intratesticular injection of the radioisotope. Groups of four rats were killed on alternate days after injection, and the outer dense fibers were isolated from the caput epididymal sperm. The radioactivity incorporated into the whole sperm and into the isolated fibers showed a sharp peak at 10 days after injection. Therefore, considering the known kinetics of spermatogenesis in the rat, the maximal incorporation of radioactivity into the fibers occurred during the second half of spermiogenesis. The radioactivity incorporated into the six major polypeptides of the fibers separated by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate accounted for 95 percent of the total radioactivity associated with the isolated fibrillar complex. Furthermore, analysis of the time-course incorporation of [3H]leucine into the polypeptides of the fibers indicated that the maximal incorporation into each of the six major components took place within the same period of time. Using two different procedures, the specific activity of each major polypeptide was determined at the time of maximal incorporation. It was found that the specific activity of the most abundant components (molecular weights of 30,400 plus 26,000) was approximately twice that of the other polypeptides.