CATALYTIC ACTIVITY OF BOVINE SEMINAL RIBONUCLEASE IS ESSENTIAL FOR ITS IMMUNOSUPPRESSIVE AND OTHER BIOLOGICAL-ACTIVITIES

Bovine seminal ribonuclease (BS-RNase) is a homologue of RNase A with special biological properties, including potent immunosuppressive acti vity. A mutant BS-RNase was created in which His-119, the active-site residue that acts as a general acid during catalysis, was changed to a n aspartic acid. H119D BS-RNase formed a dimer with quaternary structu re similar to that of the wild-type enzyme but with values of k(cat) a nd k(cat.)/K-m for the cleavage of UpA [uridylyl(3' --> 5')adenosine] that were 4x10(3)-fold lower. The mutant protein also demonstrated dra matically decreased immunosuppressive, anti-tumour, aspermatogenic, an d embryotoxic activities. The catalytic activity of BS-RNase is theref ore necessary for its special biological properties.