The accessory fibers of the sperm tailII. Their role in binding zinc in mammals and cephalopods

In mammals (bull, rat, man) and cephalopods (Eledone moschata, E. cirrhosa, Octopus vulgaris) most of the sperm zinc is concentrated in the flagellar accessory fibers (outer dense fibers). Calcium, magnesium, and copper show a more diffuse subcellular localization. This statement is based on electron probe microanalysis and on biochemical studies on fibers purified by density step centrifugation. The accessory fibers also account for the highest percentage of sulfhydryl groups in both mammalian and cephalopod sperm. Compared to other sperm structures, there is a low degree of protein cross-linking through -S-S- groups. Evidently the sulfhydryl groups are involved in the binding of zinc. Zinc binding groups other than -SH groups are also present, particularly in cephalopod fibers where high amounts of histidine are detected. The results may be related to the mechanical properties of the accessory fibers and to the action exerted by zinc on sperm motility.