INHIBITION OF RIBONUCLEOTIDE REDUCTASE BY NITRIC-OXIDE DERIVED FROM THIONITRITES - REVERSIBLE MODIFICATIONS OF BOTH SUBUNITS

Thionitrites are spontaneous nitric oxide (NO) donors in neutral aqueo us solutions. Consequently, they inhibit ribonucleotide reductase, the rate-limiting enzyme in DNA synthesis, from Escherichia coli and muri ne adenocarcinoma TA3 cells. They also inhibit tumor cell proliferatio n. Reaction of thionitrites with protein R1, the large subunit, result s in the nitrosation of cysteines, as shown from the formation of a ch romophore with a characteristic absorption at 340 nm. EPR spectroscopy both on whole murine R2-overexpressing L1210 cells and on the pure pr otein showed that the tyrosyl radical of protein R2, the small subunit , reversibly couples to the NO radical, presumably leading to nitrosot yrosine adducts. Both molecular events might be at the origin of the i nhibition of ribonucleotide reductase by NO, since a number of cystein es and the tyrosyl radical are essential for catalysis. These results identify NO donors as a new class of inhibitors of ribonucleotide redu ctase with potential applications as anticancer or antiviral chemother apy agents.