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Sperm immobilization factor (SIF) was extracted and purified from Escherichia coli filtrate. Characterization of SIF using Liquid chromatography -mass spectrometry (LC-MS) revealed its sequence similarity to chaperone protein HchA of E. coli O157:H7. Further the effect of SIF on sperm functions such as acrosome reaction was studied and the results showed that SIF (500µg/ml) significantly inhibits calcium ionophore induced acrosome reaction. SIF was also found to decrease Mg ++ ATPase activity of spermatozoa from 869.5 (control) to zero at 25µg/ml. The corresponding SIF binding receptor was also isolated and purified. The tryptic peptide analysis of SIF binding receptor by MALDI-TOF (Matrix assisted laser desorption ionization-time of flight) showed its sequence similarity to chain A structure of human apolactoferrin. Further specificity of SIF and SIF binding receptor was studied by calorimetric studies that showed binding constant (K), enthalpy of binding (ΔH°), free energy (ΔG) and entropy (ΔS) to be 1460/M and -11.7kJ/mole, -18.76 kJ/mole and 22.77 J/moleK, respectively.

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