Adenosine Triphosphate in Muscles of Rats Studied with Radioactive Phosphorus*

Immediate decreases of the concentrations of glycogen and phosphocreatine are found in continuously working muscles, and many seconds later decreases of the adenosine triphosphate are found also, The hydrolysis of the adenosine triphosphate, which involves the liberation of 2 moles of phosphate, may be detected in 40 seconds and reaches its maximumin 1 or 2 minutes in muscles working at the rate of three contractions per second (1). The concentration of adenosine triphosphate remains low as work continues and is restored to its original level slowly when rest ensues. Hydrolysis and resynthesis of adenosine triphosphate without an appreciable change of its concentration are continuously occurring in resting muscles, according to studies with radioactive phosphorus. The uptake of P32 by the acid-labile phosphate groups is fairly rapid, as shown by Hevesy and Rebbe (2, 3) and by Sacks and Altschuler (4, 5), while the uptake by the stable phosphate group is extremely slow, as shown by Korzybski and Parnas (6). Sacks did not find any change in the distribution of P32 in adenosine triphosphate from a 15 second period of isometric contraction. Lohmann (7) showed that the dephosphorylation of adenosine triphosphate is a stepwise process involving the formation of adenosine diphosphate and then of adenylic acid. The first step is catalyzed by the enzyme adenosine triphosphatase which Engelhardt and Ljubimowa (8) and others (9) have shown to be myosin; the second step is catalyzed by a more soluble enzyme, adenosine diphosphatase. We have measured the uptake of Ps2 by each of the phosphate groups in the adenosine triphosphate of muscles which were resting, working, or recovering from work. The experimental procedure involved the isolation of adenosine triphosphate, hydrolysis with myosin, and isolation of adenosine diphosphate, then hydrolysis with acid. The phosphorus content and its radioactivity for each mg. of phosphorus of each fraction hydrolyzed furnished the basis of comparison. The P32 of the adenosine triphosphate from resting muscle at the end of an hour was distributed in a proportion