Bovine seminal ribonuclease: non-hyperbolic kinetics in the second reaction strep

Binding patterns and kinetics of RNase a interaction with RNA

Kinetics and binding studies of RNase A and its natural polymeric substrate (RNA), as well as the natural mixture of free 3'-ribonucleotides, were performed by difference spectrophotometry. The obtained kinetic saturation curve, with an anomalous nonhyperbolic shape and a distinct transition point, showed the interchange between the two conformational forms of the enzyme. This occurred in a narrow range of substrate concentration. At low substrate concentration, in spite of the existence of one catalytic cleft, RNase A behaves as a cooperative system, perhaps due to the interactions among the four cooperative binding subsites in the active cleft. At high substrate concentration, the conformational change did occur and was accompanied by a decrease in cooperativity and increment of the catalytic constant. The multiphasic shape of the binding curve, which, in the presence of the enzyme, produced 3'-ribonucleotides (as the ligand molecules), shows four binding subsites. The first three subsites are specific for the attachment of phosphate, ribose, and base moieties belonging to the first bound 3'-ribonucleotide in the direction of 3'-phosphate --> ribose --> base-5'. The fourth subsite relates to the second phosphate group of the second bound 3'-ribonucleotide. The binding direction also converts to 5'-phosphate --> ribose --> base-3' for the ribonucleotide monomers in the RNA structure.

Immunosuppressive activity of bovine seminal RNase on T-cell proliferation

The interest in the immunosuppressive activity of mammalian seminal plasma depends largely on its putative role in the immunoregulation of both the male and female genital systems. We report here that the immunosuppressive action of bovine seminal plasma is based on the presence in this fluid of copious amounts of an immunosuppressive RNase, bovine seminal RNase. Studies of structure-function relationships have revealed that the immunosuppressive activity of seminal RNase depends on the integrity of the dimeric structure of the enzyme, as well as on the integrity of its catalytic function. While bovine seminal RNase has no effect on the secretion of interleukin-2 by T-cell cultures, the enzyme has been found to decrease drastically the expression of the alpha-chain of the interleukin-2 receptor on the T-cell membrane.

Dissociation and reconstitution of bovine seminal RNAase: construction of a hyperactive hybrid dimer

The quaternary structure of bovine seminal ribonuclease, the only dimeric protein in the superfamily of ribonucleases, is maintained both by noncovalent forces and by two intersubunit disulfides. The available monomeric derivatives of the enzyme may not be reassembled into dimers. They are catalytically active, but do not retain certain properties of the dimeric enzyme, such as: (i) the ability to respond cooperatively to increasing substrate concentrations in the rate-limiting reaction step; and (ii) the antitumor and immunosuppressive actions. In this report we described the preparation of stable monomers of seminal ribonuclease which can be reassociated into covalent dimers indistinguishable from the native protein. With this procedure a hybrid dimer was constructed, made up of a native subunit associated to a subunit catalytically inactivated by selective alkylation of the active site His-119. This dimer was found to have enzymic properties typical of monomeric ribonucleases, such as a hyperbolic saturation curve in the hydrolytic rate-limiting step of the reaction. However, the hybrid dimer was one order-of-magnitude more active than the dimeric enzyme.

A calorimetric approach to the study of the interactions of cytidine-3'-phosphate with bovine seminal ribonuclease

A calorimetric study at 25 degrees C is reported on the interaction between allosteric bovine seminal ribonuclease and cytidine-3'-phosphate. The results are compared with those obtained under identical experimental conditions for the interaction of pancreatic ribonuclease A and the same nucleotide. The analysis of the data provides evidence that the binding sites of seminal ribonuclease for cytidine-3'-phosphate are not equivalent, in agreement with previous equilibrium dialysis studies. A model with two sites with different affinities toward the nucleotide, the site with higher affinity resembling the binding site of pancreatic ribonuclease, is proposed. The values calculated for the thermodynamic parameters provide an insight of the forces involved in the interaction of the two enzymes with the nucleotide.

Co-operativity in seminal ribonuclease function. Kinetic studies

Kinetic studies with substrates of the hydrolytic rate-limiting reaction step revealed that the non-hyperbolic kinetics of bovine seminal RNAse may not be ascribed to microheterogeneity of the enzyme or to hysteretic effects. The substrate saturation curves with intermediate plateau and the activating and inhibiting effects of the reaction product, respectively at low and high concentrations, are explained in terms of mixed co-operativity, with binding at subsites that is a prerequisite for full activity of the enzyme. A model is proposed that is supported also by the results of binding studies.

Co-operativity in seminal ribonuclease function: binding studies

Binding of nucleotides to bovine seminal RNAase was studied by differential spectrophotometry and equilibrium dialysis. Cytidine 3'-phosphate, the reaction product of the hydrolytic, rate-limiting step of the reaction, was found to be capable, in contrast to related nucleotides, of discriminating between the two structurally identical active sites of the enzyme. Negative co-operativity, with a 'half-of-sites' reactivity, was found at lower concentrations of ligand, whereas at higher concentrations positive co-operativity was detected. These findings exclude that the non-hyperbolic kinetics previously reported for the hydrolytic step of the reaction are due to hysteretic effect. A model of mixed-type co-operativity is proposed for interpreting the binding data.

Site-directed alkylation and site-site interactions in bovine seminal ribonuclease

Active-site histidine residues of bovine seminal RNase have been found to react with bromoacetic acid and with 2'(3')-O-bromoacetyluridine (BrAcUrd) at a much faster rate than free histidine. The former reagent reacts preferentially at the pros-N of His119, the latter is specific for the tele-N of His12. Alkylation with bromoacetic acid is mutually exclusive for either His119 or His12 and takes place predominantly at His119, while with BrAcUrd alkylation was found to be selective for His12. These results are very similar to those obtained with the same reagents on RNase A, confirming that seminal and pancreatic ribonucleases have similar geometries at their active sites. On the other hand, the kinetics of reaction of bromoacetyluridine with seminal RNase reveal a 'half-of-the sites' reactivity of the enzyme for this reagent, which is found to discriminate between the two structurally identical active sites of the dimeric enzyme.

Reacquisition of quaternary structure by fully reduced and denatured seminal ribonuclease

Air-regenerated monomers of bovine seminal ribonuclease have been found capable of reassociating into native dimers, whereas monomers refolded in the presence of a glutathione redox mixture do not reassociate into dimers [Smith, K. G., D'Alessio, G. and Schaffer, S. W. (1978) Biochemistry 17, 2633-2638]. The crucial step in the process of regeneration of dimers is an isomerization step, which the newly refolded monomers undergo in order to reassociate into dimers. The two sulfhydryls at sequence positions 31 and 32 of the seminal RNAase chain, forming in the native dimer the intersubunit disulfides, have been found to have an important role in the refolding of the monomeric intermediates, as well as in the regeneration of dimers.

Comparative proton NMR studies of bovine semen and pancreas ribonucleases

The fine structure of bovine semen RNAase was studied with proton NMR spectroscopy making use of the four-protein system constituted by dimeric bovine semen RNAase, its catalytically active monomeric bis-(S-carboxymethyl-31,32) derivative, the naturally monomeric RNAase A from the pancrease of the same species, and dimerized RNAase A. Only four histidine C-2 H resonances were observed in the aromatic spectrum of bovine semen RNAase, which belong to the four histidine residues present in the sequence of bovine semen RNAase subunits at positions identical with those of the histidines of RNAase A. This is indicative of identical environments for the individual histidine residues in both subunits. These resonances were assigned (i) by comparing their titration curves with the corresponding curves obtained with RNAase A and with monomeric bovine semen RNAase and (ii) by evaluating the effects on their titration curves of nucleotide binding. Very similar NMR parameters were measured for His-105 and also for His-119 of seminal and pancreatic RNAase, while His-12 was found to have different environments in the two proteins. The distinctive NMR features of His-48 in bovine semen RNAase confirmed the role of the hinge regions of the subunits in maintaining the dimeric structure of the protein. While monomerization of the seminal enzyme reduced the differences between the histidine C-2 H resonances of RNAase A and bovine semen RNAase, dimerization of RNAase A did not affect the NMR spectrum of this protein, thus indicating as unlikely the possibility that the quaternary structure of bovine semen RNAase resembles that of dimerized RNAase A.