PORPHYRIN BIOSYNTHESIS .6. SEPARATION AND PURIFICATION OF PORPHOBILINOGEN DEAMINASE AND UROPORPHYRINOGEN ISOMERASE FROM COW LIVER . PORPHOBILINOGENASE AN ALLOSTERIC ENZYME

1. 1.|Porphobilinogenase has been isolated and purified from cow liver and its components, porphobilinogen deaminase and uroporphyrinogen isomerase, have been separated from each other and purified. 2. 2.|The effect of NH4+ was studied. The deaminase exhibited classical Michaelis-Menten kinetics in the absence or presence of NH4+, which at high concentrations behaved as a noncompetitive inhibitor of the deaminase. As expected from Hill plots, n = 1 both in the absence or presence of NH4+. Instead, when activity of porpho bilinogenase is plotted versus porphobilinogen concentration, sigmoid curves are obtained; but the presence of NH4+ at different concentrations altered the kinetic parameters of this enzymic system, again showing normal kinetics. In addition, n values were found to be 2 for porphobilinogen per porphobilinogenase molecule and 1 in the presence of NH4+ which behaves as a competitive inhibitor of the isomerase. Results are discussed in relation to the allosteric theories of monod et al.1,2 and liver porphobilinogenase is proposed to be an allosteric protein. 3. 3.|The presence of an ultrafiltrable factor which stimulates uroporphyrinogen formation from porphobilinogen has been revealed. © 1969.