Human Sperm Plasma Membranes Possess α-D-Mannosidase Activity but no Galactosyltransferase Activity1

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Abstract
Previous studies from this laboratory and others have identified several enzymes on the surface of mammalin spermatozoa. Some of these enzymes, namely a galactosyltransferase and a novel .alpha.-D-mannosidase, are believed to play a ligand-like role in recognizing and binding to the complementary moiety(ies) present on zona pellucida glycoconjugates. However, little or no information is available about the occurrence of these enzymes in human spermatozoa. In the present report, we show that a very small amount of the total galactosyltransferase activity present in human semen is associated with spermatozoa. Moreover, our failure to find a significant amount of the enzyme on sperm plasma membranes suggests that the enzyme is not associated with the sperm surface. Therefore, it is unlikely that galactosyltransferase in humans has the same ligand-like role in zona binding that is demonstrated in mouse sperm. In contrast, nearly 5% of .alpha.-D-mannosidase activity was repeatedly found in the salt-washed plasma membrane fraction. The recovery and enrichment of the .alpha.-D-mannosidase was nearly one-half that observed for adenylate cyclase and nearly one-third that for phosphodiesterase I, the two sperm plasma membrane marker enzymes. The differential enrichment and recovery of the sperm surface .alpha.-D-mannosidase is consistant with our previous studies in rat spermatozoa, and suggests that .alpha.-D-mannosidase may be localized on morphologically distinct region(s) of the sperm plasma membranes. The properties of human sperm surface .alpha.-D-mannosidase are quite similar to those reported by us for rat sperm plasma membrane mannosidase, but quite different from human sperm acid .alpha.-D-mannosidase. In addition, whereas anti-rat epididymal .alpha.-D-mannosidase antibody (IgG-fraction) cross-reacted with the human sperm acid .alpha.-D-mannosidase, no cross-reactivity was observed with the sperm surface mannosidase. A small amount of fucosyltransferase (< 1% of the enzyme originally present on spermatozoa) was found in the salt-washed plasma membrane, but the enrichment of the enzyme was only one-tenth of that observed for adenylate cyclase. The potential ligand-like role of human sperm surface .alpha.-D-mannosidase and other sperm surface enzymes during fertilization is discussed.