Comparative studies on physical properties concerned with the stability of unphosphorylated, phosphorylated and proteolytically modified L-type pyruvate kinase from pig liver

A study was made of the effects of some agents on the stability of unphosphorylated pyruvate kinase type L, the enzyme phosphorylated with cyclic 3',5'-AMP-stimulated protein kinase and the subtilisin-modified enzyme form from which the phosphorylatable site had been removed. The phosphorylated pyruvate kinase was found to be the most labile of the enzyme forms at high temperature and in the presence of urea. The circular dichroism spectrum of the phosphorylated enzyme also differed from that of the unphosphorylated and proteolytically modified forms. All three forms of the enzyme showed a high degree of stability over a wide pH range. The unphosphorylated enzyme seemed, however, to be the most sensitive to differences in pH. Only 10% of its maximal activity remained after incubation at pH 10 and 30 degrees C for 30 min, compared with 30% and 75% for the phosphorylated and proteolytically modified forms of the enzyme, respectively. Of the three enzyme forms tested the subtilisin-modified pyruvate kinase was most rapidly inactivated by trypsin. These results taken together suggest that the phosphorylated enzyme has a less ordered structure than the other two enzyme forms studied.