Classification, inhibition, and specificity studies of the vitelline coat lysin from toad sperm

AbstractThe sonicated supernatant of the sperm of the toad, Bufo japonicus, can digest easily the vitelline coat (VC) of uterine eggs, and to a lesser extent the VC of coelomic eggs, but not that of activated eggs. The VC lysis and fertilization were competitively inhibited in the presence of t‐butyloxycarbonyl‐L‐Gln‐L‐Arg‐L‐Arg‐4‐methylcoumaryl‐7‐amide (Boc‐Gln‐Arg‐Arg‐MCA), suggesting the involvement of proteases in the fertilization process. Starting from a sonicated supernatant, a potent VC lysin, possessing hydrolytic activity on Boc‐Gln‐Arg‐Arg‐MCA, was obtained by anion‐exchange chromatography and gel filtration. The activity of the partially purified lysin was inhibited by diisopropyl fluorophosphate (DFP) and by such trypsin inhibitors as soybean trypsin inhibitor, leupeptin, and (p‐amidinophenyl) methanesulfonyl fluoride hydrochloride, but not by chymostatin, E‐64, and ethylene glycol bis(β‐aminoethyl ether)‐N,N,N′,N′‐tetraacetic acid. The molecular weight of the lysin was estimated to be 32K, based on the fluorographic image of 3H‐DFP binding to the lysin on sodium dodecyl sulfate gel electrophoresis. The VC lysin was most active at pH 7.0–7.6 and under low ionic strength equivalent to fresh water. The release of the VC lysin was induced upon incubation of sperm with the contents of oviducal pars recta granules (PRG), which are known to induce the acrosome reaction. We conclude that the protease studied here represents the VC lysin of toad sperm that is involved in fertilization by digesting the VC of uterine eggs, probably released as a result of the acrosome reaction induced by PRG.