Kim J S, Soucek J, Matousek J, Raines R T
Department of Biochemistry, University of Wisconsin, Madison 53706-1569, USA.
J Biol Chem. 1995 May 5;270(18):10525-30. doi: 10.1074/jbc.270.18.10525.
Bovine seminal ribonuclease (BS-RNase) is a homolog of RNase A with special biological properties that include specific antitumor, aspermatogenic, and immuno-suppressive activities. Unlike RNase A, BS-RNase is a dimer cross-linked by disulfide bonds between Cys31 of one subunit and Cys32 of the other. At equilibrium, this dimer is a mixture of two distinct quaternary forms, M = M and M x M. The conversion of M = M to M x M entails the exchange of NH2-terminal alpha-helices between subunits. Here, the cytotoxic activities of purified M x M were shown to be greater than those of purified M = M, despite extensive equilibration of M = M and M x M during the time course of the assays. Replacing Cys31 or Cys32 with a serine residue did not compromise the enzymatic activity of dimeric BS-RNase, but reduced both the fraction of M x M at equilibrium and the cytotoxicity. We conclude that the M x M form is responsible for the special biological properties of BS-RNase. Since cytosolic ribonuclease inhibitor binds tightly to monomeric but not dimeric BS-RNase and only the M x M form can remain dimeric in the reducing environment of the cytosol, we propose that BS-RNase has evolved its M x M form to retain its lethal enzymatic activity in vivo.
牛精核糖核酸酶（BS-RNase）是核糖核酸酶A的同源物，具有特殊的生物学特性，包括特异性抗肿瘤、抗生精和免疫抑制活性。与核糖核酸酶A不同，BS-RNase是一种二聚体，通过一个亚基的Cys31与另一个亚基的Cys32之间的二硫键交联。在平衡状态下，这种二聚体是两种不同四级形式M = M和M×M的混合物。M = M向M×M的转变需要亚基之间NH2末端α-螺旋的交换。在这里，尽管在测定过程中M = M和M×M有广泛的平衡，但纯化的M×M的细胞毒性活性显示大于纯化的M = M。用丝氨酸残基取代Cys31或Cys32不会损害二聚体BS-RNase的酶活性，但会降低平衡时M×M的比例和细胞毒性。我们得出结论，M×M形式负责BS-RNase的特殊生物学特性。由于胞质核糖核酸酶抑制剂与单体BS-RNase紧密结合，但不与二聚体BS-RNase结合，并且只有M×M形式在胞质溶胶的还原环境中可以保持二聚体状态，我们提出BS-RNase进化出其M×M形式以在体内保留其致死性酶活性。
