De novo purine synthesis in avian liver. Co-purification of the enzymes and properties of the pathway.

The enzymes of the de novo purine biosynthetic pathway have been partially co-purified from pigeon liver by a method dependent upon the use of the nonionic polymer polyethylene glycol for enzyme stabilization and cofractionation. Although the enzymes did not appear to constitute a large macromolecular complex it was evident that some particular inter-relationship between them was preserved during the purification procedure. Analysis of the end products and pathway intermediates was carried out primarily by sensitive high pressure liquid chromatographic techniques. Substrate and cofactor requirements were confirmed and optimal conditions of pH, temperature, and K+ ion activation established. At phosphoribosyl pyrophosphate (PP-ribose-P) concentrations below 0.3 mM the activity of the first pathway enzyme amidophosphoribosyltransferase was rate-limiting, and the inhibition of this enzyme by AMP regulated the rate of purine ring synthesis. At higher concentrations of PP-ribose-P, aminoimidazole ribonucleotide synthetase, the fifth enzyme of the pathway became rate limiting and was subject to inhibition by added AMP. It was evident that the regulation of purine synthesis was quite complex and that AMP inhibition (perhaps reflected in a low adenylate energy charge) can be effected at different points on the purine pathway.