ENZYMATIC CHANGES OF THE BOVINE PITUITARY MULTICATALYTIC PROTEINASE COMPLEX, INDUCED BY MAGNESIUM-IONS

The effect of magnesium ions on the catalytic activities of the bovine pituitary multicatalytic proteinase complex (MPC) was studied. Mg2+ markedly stimulated the breakdown of dephosphorylated β-casein (caseinolytic activity) and the hydrolysis of CbzLeuLeuGlu-2-naphthylamide (peptidylglutamyl peptide bond hydrolyzing activity) by a 1700-fold purified preparation of MPC. Cleavage of CbzD-AlaLeuArg-2-naphthylamide (trypsin-like activity) was strongly inhibited and cleavage of CbzGlyGlyLeup-nitroanilide (chymotrypsin-like activity) was weakly inhibited. Similar results were produced when enzymatic activities in the absence of Mg2+ were measured at 52 °C rather than at 37 °C. Trace protein impurities were removed by phenyl-Sepharose chromatography. This additional chromatographic step, while not changing the specific activities of hydrolysis of the three synthetic chromogenic substrates, led to a marked activation of the breakdown of dephosphorylated β-casein. Mg2+ was not able to further stimulate the caseinolytic activities of either the phenyl-Sepharose-treated preparation or the preparation measured at 52 °C. Mg2+ therefore converts a "repressed" form of MPC to an "activated" form, possibly by promoting dissociation of a protein inhibitor, and may serve as a physiological regulator of this enzyme complex. © 1992.