Pigeon Liver Amidophosphoribosyltransferase

Amidophosphorihosyltransferase (EC 2.4.2.14) has been partially purified from pigeon liver, and ligand-induced alterations in molecular and kinetic properties have been studied. In Tris-HCl buffer the predominant form of’ the enzyme has an sZO,w of 5.9 * 0.7, Stokes radius of 42 A, and estimated molecular weight of 102,000. Incubation with phosphoribosylpyrophosphate (PP-ribose-P) results in an increase in the s20.w to 7.9 * 0.6. Stokes radius to 53 A, and estimated molecular weight to 172,000. Incubation of this larger form with purine rihonucleotides leads to a decrease in the molecular weight of amidophosphoribosyltransferase that is proportional to the concentration of purine ribonucleotide. Purine ribonucleotides produce sigmoidal kinetics with respect to the substrate PP-rihose-P, with Hill coefficients of 1.4 to 1.6 and 1.8 to 2.0 in the presence of AMP and GMP, respectively. Incubation with 0.6 M KC1 leads to sigmoidal kinetics, Hill coefficient of 1.8, and dissociation of the larger form of amidophosphorihosyltransferase. Inorganic phosphate has complex effects upon the enzyme. In 25 IIIM potassium phosphate buffer the enzyme aggregates to a large form with an sza+ of 8.3 L 0.2, Stokes radius of 53 A, and estimated molecular weight of 181,000. Inorganic phosphate and PP-ribose-P both stabilize the enzyme to storage in vitro at 4”. However, inorganic phosphate is 4 times more effective than PP-rihose-P in preventing inactivation of the enzyme by sodium dodecyl sulfate. Inorganic phosphate produces sigmoidal kinetics with respect to PP-ribose-P, Hill coefficient of 1.5. The interaction coefficients for AMP and GMP are reduced from 1.8 to 1.2 and 2.2 to 1.4, respectively, in the presence of 25 mM potassium phosphate. It is concluded that pigeon liver amidophosphoribosyltransferase is a complex allosteric protein whose activity is regulated by a series of conformational changes induced by a number of ligands.