STRUCTURAL BASIS FOR THE BIOLOGICAL-ACTIVITIES OF BOVINE SEMINAL RIBONUCLEASE

Bovine seminal ribonuclease (BS-RNase) is a homolog of RNase A with sp ecial biological properties that include specific antitumor, aspermato genic, and immunosuppressive activities. Unlike RNase A, BS-RNase is a dimer cross-linked by disulfide bonds between Cys(31) of one subunit and Cys(32) Of the other. At equilibrium, this dimer is a mixture of t wo distinct quaternary forms, M=M and MxM. The conversion of M=M to Mx M entails the exchange of NH2-terminal alpha-helices between subunits. Here, the cytotoxic activities of purified MxM were shown to be great er than those of purified M=M, despite extensive equilibration of M=M and MxM during the time course of the assays. Replacing Cys(31) or Cys (32) with a serine residue did not compromise the enzymatic activity o f dimeric BS-RNase, but reduced both the fraction of MxM at equilibriu m and the cytotoxicity. We conclude that the MxM form is responsible f or the special biological properties of BS-RNase. Since cytosolic ribo nuclease inhibitor binds tightly to monomeric but not dimeric BS-RNase and only the MxM form can remain dimeric in the reducing environment of the cytosol, we propose that BS-RNase has evolved its MxM form to r etain its lethal enzymatic activity in vivo.