Interchain Disulfide Bridges in Ribonuclease BS-1

RNAase BS-1, a dimeric ribonuclease isolated from bovine seminai plasma, is made up of two identica!subunits whose amino acid sequence is homologous to the sequence of bovine pancreatic RNAase A. The dimeric structure, resistant to denaturating agents, is sensitive to thiol reagents even in the absence of denaturants. The isolation and characterization of a cystine peptide containing two adjacent cystine residues is reported. As the peptide molecular weight is halved after reductive cleavage with dithio­ threitol, a struture based on two interchain disulfide bonds between the two adjacent cystine of each subunit is proposed. The singularity of such a structure for a small enzymatic protein is discussed.