Inhibition of ribonucleotide reductase by antitumor agents related to levodopa and dopamine.

Using partially purified enzyme from L1210 cells, dihydroxybenzene derivatives related structurally to dopamine were shown to reversibly inactivate ribonucleotide reductase. A structure-activity analysis revealed that derivatives with side-chains, which contain a negatively-charged group, had significantly reduced inhibitory activity. The ability of these compounds to inhibit ribonucleotide reductase was dependent on the hydroxyl groups being in the ortho position and did not correlate with free radical inhibitory activity. A kinetic analysis by the method of Lineweaver-Burk indicated that the inhibition of ribonucleotide reductase by the derivative 3,4-dihydroxybenzylamine was competitive with the reducing substrate dithioerythritol. This analog, in combination with hydroxyurea, gave synergistic inhibition or ribonucleotide reductase, suggesting different sites of action. Using Tween 80-treated L1210 cells, it was found that these drugs had an immediate inhibitory effect on ribonucleotide reductase activity in intact, reversibly permeabilized cells. Furthermore, although these drugs had no immediate effect on DNA polymerase, in permeabilized L1210 cells (when the cells were preincubated with the dihydroxybenzene derivatives for 1 hr prior to permeabilization), there was significant inhibition of DNA polymerase activity. The two key enzymes for DNA synthesis appear to be sequentially inhibited by these analogs, with the reduced form (quinol) inhibiting ribonucleotide reductase and the oxidized form (quinone) inhibiting DNA polymerase.