An assessment of proteolytic enzymes in Tetrahymena thermophila.

Cellular extracts of Tetrahymena thermophila were found to contain substantial levels of proteolytic activity. Protein digestion occurred over broad ranges of pH, ionic strength, and temperature and was stimulated by treatment with thiol reductants, EDTA and sodium dodecyl sulfate. Incubation at temperatures > or = 60 degrees C or with high concentrations of chaotropic reagents such as 10 M urea or 6 M guanidine-HCl caused an apparent irreversible loss of activity. Activity was also strongly diminished by increasing concentrations of divalent cations. Several peptide aldehydes, p-hydroxymercuribenzoate, and alkylating reagents such as iodoacetate, N-tosyl-L-lysine chloromethyl ketone, N-tosyl-L-phenylalanine chloromethyl ketone, N-methylmaleimide, and trans-epoxysuccinyl-L-leucylamido-(4-guanidino)-butane were potent inhibitors of proteolytic activity. Aprotinin diminished activity by approximately 40% while benzamidine, 3,4-dichlorosocoumarin, and trypsin inhibitors from soy bean, lima bean, and chicken egg caused relatively modest inhibition of proteolytic activity. Phenylmethanesulfonyl fluoride had no apparent effect. Electrophoretic separation of proteins on SDS-polyacrylamide gels copolymerized with gelatin substrate revealed that at least eight active proteolytic enzymes were present in cell extracts ranging in apparent molecular weight from 45,000 to 110,000. Five of these apparent proteases were detected in 70% ammonium sulfate precipitates. Gelatinase activity was not detectable when extracts were pretreated with iodoacetate or E-64, indicating that all of the enzymes observed in activity gels were sensitive to thiol alkylation. Cellular extracts of T. thermophila appeared to contain multiple forms of proteolytic enzymes which were stimulated by thiol reductants and inhibited by thiol modifying reagents. Accordingly, the proteolytic enzymes present in cell extracts appear to be predominantly cysteine proteinases.

Gossypol inhibition of acrosin and proacrosin, and oocyte penetration by human spermatozoa.

Gossypol, a known antispermatogenic agent, was found to effectively inhibit the highly purified boar sperm proacrosin-acrosin proteinase enzyme system by irreversibly preventing the autoproteolytic conversion of proacrosin to acrosin and reversibly inhibiting acrosin activity. The agent appears to prevent the self-catalyzed by not the acrosin-catalyzed activation of proacrosin. In additional experiments, brief exposure of human semen to concentrations of gossypol, which did not visibly alter spermatozoal motility or forward progression, was found to irreversibly inhibit the conversion of proacrosin to acrosin although the activity of the nonzymogen acrosin was not decreased, and also to prevent the human spermatozoa from penetrating denuded hamster oocytes. Gossypol inhibition of proacrosin conversion to acrosin closely paralleled the decline in oocyte penetration. Racemic (+/-) gossypol was equally as effective as the enantiomer (+) gossypol. The results suggest that the inhibition of proacrosin conversion to acrosin is a mechanism by which gossypol exerts its antifertility effect at nonspermicidal concentrations and that low levels of gossypol should be tested for their contraceptive action when placed vaginally.

Acrosin inhibition. Comparisons of membrane-associated and -solubilized enzyme.

Acrosin is an extrinsic membrane proteinase from spermatozoa which functions in the fertilization process. Liposomes were utilized as a model system to determined possible effects of membrane association on acrosin's enzymatic activity. By comparison with solubilized enzyme, liposome-bound acrosin had a substantial reduction in the apparent affinity for "progressive" inhibitors such as leupeptin, lima bean trypsin inhibitor, soy bean trypsin inhibitor, and for a proteinase inhibitor from sperm extracts. In contrast, the liposome-bound and -solubilized enzymes were essentially identical with respect to the binding of benzamidine and p-aminobenzamidine which are competitive acrosin inhibitors. These results suggest membrane association can influence some but not all of acrosin's enzymatic properties.

Association of proacrosin with phospholipid membranes.

Proacrosin, the zymogen precursor of acrosin, was shown to associate with anionic phospholipid membranes through apparent electrostatic charge interactions. This association was diminished by elevated cation concentrations and was dependent on membrane composition, as shown both by direct binding assays and by following the phospholipid stimulation of conversion of proacrosin into acrosin.

Boar acrosin. Association of an endogenous membrane proteinase with phospholipid membranes.

Acrosin, an enzyme required for fertilization, is an endogenous proteinase associated with membranes of the sperm acrosome. Liposomes were utilized as a model system to evaluate the mode of association between highly purified boar acrosin and phospholipid bilayer membranes. Acrosin was observed to bind to liposomes containing acidic phospholipids such as phosphatidylglycerol, cardiolipin, and phosphatidylserine. There was no apparent binding of acrosin to liposomes which consisted of nonacidic phospholipids, thus indicating that an ionic phospholipid constituent was required for binding. Increased ionic strength caused a significant reduction in acrosin-liposome association with an inverse effect on enzyme-membrane dissociation. Acrosin-liposome association and dissociation were similarly effected by increasing concentrations of divalent cations at constant ionic strength, and by reductions in pH. Equilibrium binding experiments, with anionic liposomes, suggest the presence of either multiple classes of independent binding sites, or apparent negative cooperativity, with a range in the apparent affinity constant (Ka) from 2 x 10(11) M-1 at low acrosin concentrations to 3 x 10(8) M-1 at high acrosin concentrations. The membrane-associated enzyme was accessible to concanavalin A, and to high molecular weight substrates, demonstrating that a portion of the acrosin molecule is exposed at the membrane surface. In addition, acrosin binding to liposomes had no apparent effect of hydrolysis of soluble protein or synthetic substrates. The results demonstrate that acrosin-liposome binding is due in part to electrostatic charge interactions and indicate that the enzyme has properties of an extrinsic membrane protein.

Structure-activity relationships for the inhibition of acrosin by benzamidine derivatives.

A series, consisting of 52 benzamidine derivatives, was evaluated for inhibitory activity against homogeneous boar sperm acrosin. All of the compounds in the series proved to be more potent than benzamidine (Ki = 4.0 x 10(-6) M), with one of the derivatives, alpha-(4-amidino-2,6-diiodophenoxy)-3-nitrotoluene (compound 16), showing outstanding potency with a Ki value of 4.5 X 10(-8) M. Although all of the derivatives were effective acrosin inhibitors, structural specificity was observed within homologous groups of compounds. The information gained from this preliminary study should prove extremely beneficial in the design and synthesis of future acrosin inhibitors.