Porphyrin biosynthesis: VI. Separation and purification of porphobilinogen deaminase and uroporphyrinogen isomerase from cow liver. Porphobilinogenase an allosteric enzyme

Abstract

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 NH 4 + was studied. The deaminase exhibited classical Michaelis-Menten kinetics in the absence or presence of NH 4 +, 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 NH 4 +. Instead, when activity of porpho bilinogenase is plotted versus porphobilinogen concentration, sigmoid curves are obtained; but the presence of NH 4 + 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 NH 4 + 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.