Ribonuclease A: Disulfide Bonds, Conformational Stability, and Cytotoxicity

Disulfide bonds between the side chains of cysteine residues are the only common crosslinks in proteins. Bovine pancreatic ribonuclease A (RNase A) is a 124-residue enzyme that contains four interweaving disulfide bonds (Cys26-Cys84, Cys40-Cys95, Cys58-CysllO, and Cys65-Cys72) and catalyzes the cleavage of RNA. The contribution of each disulfide bond to the confonnational stability and catalytic activity of RNase A was detennined using variants in which each cystine was replaced independently with a pair of alanine residues. Of the four disulfide bonds, the Cys40-Cys95 and Cys65-Cys72 cross-links are the least important to confonnational stability. Removing these disulfide bonds leads to RNase A variants that have Tm values below that of the wild-type enzyme but above physiological temperature. Unlike wild-type RNase A. G88R RNase A is toxic to cancer cells. To investigate the relationship between conformational stability and cytotoxicity, the C40AlC95A and C65A1C72A variants were made in the G88R background. Also, a new disulfide bond was introduced into G88R RNase A and a variant missing the Cys65-Cys72 disulfide bond. The Tm values of the four disulfide variants of G88R RNase A vary by nearly 30°C. The confonnational stability correlates directly with cytotoxicity as well as with resistance to proteolysis. These data indicate that conformational stability is a key determinant of RNase A cytotoxicity and suggest that cytotoxicity relies on avoiding proteolysis. This finding suggests a means to produce new cancer chemotherapeutic agents based on mammalian ribonucleases. To be cytotoxic, ribonucleases must enter the cytosol and degrade cellular RNA. The cytosolic ribonucleolytic activity of RNase A is limited, however, by the presence of excess