The level of phosphorylation of nucleotides as related to the incorporation of [6- 14C]orotic acid into ribonucleic acid by rat-liver homogenates

Abstract

1. 1. Evidence is presented to support the hypothesis that in rat-liver homogenates ATP inhibits the incorporation of radioactive uridine nucleotides into RNA. 2. 2. Increasing the concentration of MgCl 2 from 3 m M to 6 m M partially reverses inhibition by ATP. Higher concentrations of magnesium markedly inhibit incorporation regardless of the ATP concentration. 3. 3. Equilibrium between mono-, di-, and triphosphates is established at a level of 2.2-2.8 phosphates/adenosine by 20 min after the addition of ATP, ADP, or a mixture of AMP and ATP. It is only after rapid dephosphorylation of nucleotides has begun that significant incorporation of orotic acid into RNA is observed. 4. 4. When dephosphorylation of adenine nucleotides is almost complete, the specific activity of RNA reaches a maximum. Then the specific activity decreases, indicating that radioactive nucleotides are hydrolyzed from RNA faster than non-radioactive nucleotides.