Enzymatic synthesis of 5'-phosphoribosylamine from ribose 5-phosphate and ammonia, an alternate first step in purine biosynthesis.

An enzyme activity responsible for the formation of 5’phosphoribosylamine from ribose 5-phosphate and a nitrogen donor was prepared from livers of ducks, chickens, and pigeons. This enzyme activity is distinct from ribosylpryophosphate 5-phosphate amidotransferase (EC 2.4.2.14), which catalyzes the formation of 5’-phosphoribosylamine from ribosylpyrophosphate 5-phosphate and glutamine. The enzyme was partially purified and completely separated from ribosylpyrophosphate 5-phosphate amidotransferase. For maximal synthesis of 5’-phosphoribosylamine, the reaction required ribose 5-phosphate, ammonium chloride, ATP, and magnesium ions. Prolonged heating at 80” destroyed the activity of the enzyme. It is proposed that an alternate pathway for the initial step of purine synthesis exists, namely, the direct conversion of ribose 5-phosphate to 5’-phosphoribosylamine. ‘Ihis reaction is enzymatic and is sensitive to inhibition by ribonucleotides and by ribose 5-phosphate. Goldthwait et al. (5) proved that the enzyme system which catalyzed Reaction 1 was separate and distinct from the enzyme systems which catalyzed Reactions 2 and 3. The enzyme system which catalyzed Reaction 1 was in the precipitate after fractionation of the soluble extract by acidification to pH 5.5, whereas the enzyme systems which catalyzed Reactions 2 and 3 were ii? the supernatant fraction. The supernatant fraction lacked the capacity to catalyze the synthesis of PP-ribose-P and of PRG from ribose-5-P, glutamine, and ATP. Hartman, Buchanan, and Levenberg (2, 6, 7) found that PP-ribose-P was a more immediate precursor of the phosphoribosyl group of PRG than was ribose-5-P, and they purified PP-ribose-P amidotransferase, the enzyme which catalyzes Reaction 2. PRA synthesis, catalyzed by PP-ribose-P amidotransferase, has therefore been considered the first step in the biosynthesis of purines and the only source of PRA available to this pathway.