More importantly, heparin itself competes with lectins for binding to the sperm surface. Moreover, bicarbonate and heparin-induced capacitation reduced carbohydrate-binding properties of the bovine sperm surface (this study). Indeed, capacitated sperm had a lower binding rate for fucose-BSA-biotin, using a flow cytometric assay) than did uncapacitated sperm. We were able to repeat this result not only for fucose-PAA-biotin but also for LeA and heparin. The effect, however, was solely a result of bicarbonate and was not dependent on the presence of heparin or glucose.
Therefore, bicarbonate seems responsible for the reduced fucose-binding capacity. We demonstrated a high affinity of the sperm cells for mannan-biotin in the absence of heparin (both in the flow cytometric assay as well as on immobilized SPM). Only a partial decrease in mannan-biotin affinity was observed after capacitation in the presence of heparin. This supports the hypothesis that mannose (man-nan is a polymere of this sugar) plays a role in sperm-zona binding and the absence of mannan binding would distort this important prefertilization event.