REDUCTION OF SPERM CHOLESTEROL PHOSPHOLIPID RATIO IS A POSSIBLE MECHANISM FOR ENHANCEMENT OF HUMAN SPERM BINDING TO THE ZONA-PELLUCIDA FOLLOWING INCUBATION WITH PHOSPHATIDYLCHOLINE LIPOSOMES/

TEST (TES (N-tris[hydroxymethyl]methyl-2-aminoethanesulfonic acid) and Tris) yolk buffer (TYB) has recently been shown to improve the bindin g capacity of spermatozoa to zona pellucidae. The present study had tw o objectives: 1) to elucidate which component(s) of TYB dominates this effect and 2) to define the responsible mechanism. Sperm samples obta ined from subfertile men were incubated for 2 h in either TYB or media containing egg yolk lipoproteins or phospholipids. After incubation, sperm binding was tested by the hemizona assay. Yolk lipoprotein-treat ed spermatozoa bound hemizonae with efficiency equal to that of the sp ermatozoa incubated in control medium. Conversely, incubation of sperm atozoa in media containing either TYB, yolk-phospholipids, or pure pho sphatidylcholine (PC) resulted in a 2- to 3-fold increased binding cap acity (p < 0.01). A close correlation was found between the effect of yolk-phospholipids and TYB on the binding capacity of the same sperm s amples, compared to spermatozoa incubated in control medium. Incubatio n of spermatozoa in yolk phospholipid medium caused a dose-dependent i ncrease of sperm binding capacity (p < 0.05). Treatment of sperm sampl es with 1 mg/ml or more of purified PC preparation also resulted in a reduction of the sperm cholesterol:phospholipid molar ratio. Significa nt correlations between the effects of the treatments on sperm cholest erol: phospholipid molar ratio and sperm binding were obtained with yo lk-phospholipids (r = -0.55) or 1 mg/ml purified PC (r = -0.61). We co nclude that 1) the enhanced binding capacity of human spermatozoa foll owing TYB treatment is probably due to yolk-phospholipids, mainly egg yolk PC; and 2) it appears that the enhanced binding capacity of human spermatozoa following treatment with egg yolk-containing media may be a result of the reduction of the cholesterol:phospholipid molar ratio in the sperm cells.