FUCOIDIN BINDING-ACTIVITY AND ITS LOCALIZATION ON HUMAN SPERMATOZOA

We previously reported that fucoidin (a polymer of predominantly sulfa ted L-fucose) significantly inhibits: (1) tight binding of human sperm to human zona pellucida in vitro and (2) stimulation of the acrosome reaction by acid solubilized human zona pellucida. Here, we determined fucoidin binding activity on human spermatozoa and its localization o n both live and permeabilized human sperm populations. A typical bindi ng curve was demonstrated with biotinylated fucoidin. In competitive i nhibition assays with unlabelled fucoidin or human sperm membrane extr acts, IC50's were 4.0 mug/ml and 31.4 mug/ml, respectively. Fucoidin b inding was localized over the acrosomal region of methanol-fixed human sperm and this pattern of binding significantly decreased from 92 +/- 3% to 74 +/- 6% with calcium ionophore A23187 treatment (p < 0.01). B inding of fucoidin-coated beads to live (non-permeabilized) human sper m was less than 1%. Addition of the detergent, Triton-X, to permeabili ze sperm membranes resulted in a significant increase in binding (p = 0.001). These results provide evidence for the presence of a fucoidin binding compound in human spermatozoa that is localized to the membran es of the acrosomal region and can be extracted by a mild detergent ex traction. Absence of binding by fucoidin to intact but not permeabiliz ed spermatozoa suggests that the heteropolysaccharide binds to a recep tor within the acrosomal matrix. However, further investigation is war ranted to determine whether a fucoidin binding site is present both at the sperm's surface for the initial contact with the zona pellucida, and also for secondary binding after exposure of the acrosomal membran es.