Localization of Zinc in a Dense Fiber-Connecting Piece Fraction of Rat Sperm Tails Analogous Chemically to Hair Keratin

The intracellular distribution of zinc in mature rat sperm was examined by subcellular fractionation and atomic absorption spectrophotometry. The results indicate that >90 percent of this trace metal is located in the tail, ca. 85 percent in -S-S-crosslinked tail structures which do not disintegrate in 1 percent sodium dodecyl sulfate (SDS). Association of zinc with -SH is supported by the primary localization of the latter within the same SDS-insoluble material. The concentrations of both zinc and -SH decrease in the heads and tails of rat sperm during passage through the epididymis. Of the minor fraction of zinc in the rat sperm head (5-10 percent), >75 percent is not extracted with SDS and therefore appears also to be associated with -S-S-bonded structures. Over 75 percent of the zinc in isolated tails is retained by a subfraction which consists mainly of dense fibers, with connecting pieces present as a minor constituent, and is largely non-dialyzable upon solubilization of these structures. The major component isolated from the solubilized product, following aminoethylation, is of 35,000 mol. wt. Polyacrylamide gel electrophoresis in SDS also reveals significant components of ca. 75,000, 25,000 and 15,000 mol. wt. and trace components of 90,000, 70,000 and 50,000 mol wt. The amino acid composition of this protein mixture includes 11-12 percent cysteine and is found to be strikingly similar to the overall compositions of wool and guinea pig hair, which also possess sizeable contents of stably-bound zinc. The possibilities that the proteins of hair and sperm keratin (i.e.- kerateines ) share a common phylogenetic origin and undergo similar interactions with zinc during macromolecular assembly are therefore noted.