The purine nucleotide cycle. A pathway for ammonia production in the rat kidney.

Particle-free extracts prepared from kidney cortex of rat catalyze the formation of ammonia via the purine nucleotide cycle. The cycle generates ammonia and fumarate from aspartate, using catalytic amounts of inosine monophosphate, adenylosuccinate, and adenosine monophosphate. The specific activities of the enzymes of the cycle are 1.27+/-0.27 nmol/mg protein per min (SE) for adenoylosuccinate synthetase, 1.38+/-0.16 for adenylosuccinase, and 44.0+/-3.3 for AMP deaminase. Compared with controls, extracts prepared from kidneys of rats fed ammonium chloride for 2 days show a 60% increase in adenylosuccinate synthetase and a threefold increase in adenylosuccinase activity, and a greater and more rapid synthesis of ammonia and adenine nucleotide from aspartate and inosine monophosphate. Extracts prepared from kidneys of rats fed a potassium-deficient diet show a twofold increase in adenylosuccinate synthetase and a threefold increase in adenylosuccinase activity. In such extracts the rate of synthesis of ammonia and adenine nucleotide from aspartate and inosine monophosphate is also increased. These results show that the reactions of the purine nucleotide cycle are present and can operate in extracts of kidney cortex. The operational capacity of the cycle is accelerated by ammonium chloride feeding and potassium depletion, conditions known to increase renal ammonia excretion. Extracts of kidney cortex convert inosine monophosphate to uric acid. This is prevented by addition of allopurinol of 1-pyrophosphoryl ribose 5-phosphate to the reaction mixture.