Sperm motility inhibiting factor (SMIF) - A plasmatic peptide with multifunctional biochemical effects on boar spermatozoa

The sperm motility inhibiting factor (SMIF) isolated from boar seminal plasma is a peptide of 5.7 kDa. Immunoblotting assay with specific SMIF antibodies has demonstrated that this peptide is a component of 54 kDa glycoprotein-complex (especially 15 kDa protein), which has been purified from boar vesicular fluid and seminal plasma. SMIF causes immediate inhibition of sperm motility without affecting the integrity and permeability of the plasmalemma, as assessed by in vitro fluorescent probes. The aim of this study was to monitor plasmalemma changes at different regions of boar spermatozoa after incubation with SMIF. Biochemical markers were used to explain this phenomenon. A rapid decrease in ATP content and energy charge of adenylate nucleotides in the spermatozoa of different species was observed after incubation with SMIF. The suppressive effect of SMIF on lipid peroxidation and trypsin activity was dose-dependent. There was a noticeable decrease in the intensity of H-3-actinomycin D (H-3-AMD) binding to sperm chromatin. The disturbances in the susceptibility of the plasmalemma at the mid-piece region were manifested by leakage of aspartate aminotransferase (EC 2.6.1.1., AspAT). The SMIF appears to induce changes in the molecular organization of the plasmalemma and as a result affects the energy charge and biochemical functions of spermatozoal structure. The binding of SMIF with 15 kDa protein is probably related to the function of plasma capacitation factors.