Changes in the structure and catalytic activities of the bovine pituitary multicatalytic proteinase complex following dialysis.

The multicatalytic proteinase complex (proteasome) contains at least four distinct active sites catalyzing the degradation of selected chromogenic substrates (trypsin-like, chymotrypsin-like, and peptidylglutamyl peptide hydrolyzing activities) and proteins such as beta-casein. Oxidized insulin B chain was recently proposed as a model substrate for protein degradation by the multicatalytic proteinase complex (Dick, L. R., Moomaw, C. R., DeMartino, G. N., and Slaughter, C. A. (1991) Biochemistry 30, 2725-2734). We studied the dialysis-induced activation of the hydrolysis of oxidized insulin B chain by this enzyme. Removal of EDTA from purified preparations of bovine pituitary multicatalytic proteinase complex by dialysis against Tris-HCl buffers led to marked changes in the catalytic properties and structure of the enzyme. Dialysis produced a time-dependent activation of oxidized insulin B chain hydrolysis with predominant cleavage at the Glu13-Ala14 bond. A new chromogenic assay was developed for measurement of this activity. Activation was accompanied by a virtually total inactivation of the chymotrypsin-like, trypsin-like, and peptidylglutamyl peptide hydrolyzing activities. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed a loss of the 24-kDa subunit and the appearance of a new band at 21 kDa. Amino-terminal amino acid analysis established that the 21-kDa band was autolytically derived from the 24-kDa subunit. Evidence for partial dissociation and/or aggregation indicated that autolysis destabilizes the complex. By altering the profile of catalytic activities of the multicatalytic proteinase complex, autolysis may serve as a mechanism for regulation of this macromolecule.