Role of spermine in mammalian sperm capacitation and acrosome reaction

The binding properties of seminal polyamines to ram spermatozoa and their possible role in sperm capacitation and the acrosome reaction were studied. Binding and release of [14C]spermine from ram spermatozoa occurred at a rate faster than in somatic cells and were not energy-dependent. Release of bound spermine was further facilitated by heparin, a constituent of the female reproductive tract which was reported to induce capacitation and the acrosome reaction. High- and low-affinity polyamine-binding sites were identified, of which the high-affinity site was specific to polyamines with three or more amino groups. We also found that spermine inhibited the acrosome reaction and propose that it is the major seminal decapacitating factor. Since precise timing of capacitation and the acrosome reaction are critical for successful fertilization, it is suggested that the role of seminal spermine is to prevent premature capacitation and the acrosome reaction.