Initial events in the degradation of soluble cellular enzymes: factors affecting the stability and proteolytic susceptibility of fructose-1,6-bisphosphate aldolase.

The effects of disulfides (oxidized glutathione or cystine) and of cellular proteinases on rabbit muscle aldolase activity, thermal stability and susceptibility to proteolysis were determined. Native aldolase was reversibly inactivated by cystine and oxidized glutathione. Disulfide-inactivated aldolase had a lower transition temperature and enthalpy for denaturation than the native enzyme and was extensively degraded by lysosomal enzymes or a metallo-proteinase, meprin. Native aldolase was also inactivated by lysosomal enzymes or meprin; this inactivation was due to limited proteolysis in the C-terminus. However, aldolase inactivated by limited proteolysis had the same thermal stability as native aldolase and was resistant to extensive proteolysis by lysosomal enzymes or meprin. These data provide insight into the molecular basis whereby formation of mixed disulfides between proteins and glutathione or cysteine may result in unstable protein conformations and may be an initial event in the process of degradation of soluble cellular enzymes to amino acids and small peptides.