Mechanism of inhibition of mammalian ribonucleotide reductase by the iron chelate of 1-formylisoquinoline thiosemicarbazone. Destruction of the tyrosine free radical of the enzyme in an oxygen-requiring reaction.

The iron chelate of 1-formylisoquinoline thiosemicarbazone is one of the most potent inhibitors known for mammalian ribonucleotide reductase. In this study, we show that the target for the drug is the tyrosine free radical of the M2 subunit of the enzyme. The radical is destroyed by the drug in a reaction which requires oxygen. After removal of the drug, the tyrosine radical and ribonucleotide reductase activity can be regenerated by incubation of the enzyme with dithiothreitol. We propose that the iron chelate of the drug binds at the active site of the enzyme, and then the ferrous form of the chelate reacts with molecular oxygen in a redox process that, via a 1-electron reduction, leads to destruction of the M2 tyrosine radical.