Insight into the mechanism of inactivation of ribonucleotide reductase by gemcitabine 5'-diphosphate in the presence or absence of reductant.

Gemcitabine 5'-diphosphate (F(2)CDP) is a potent inhibitor of ribonucleotide reductases (RNRs), enzymes that convert nucleotides (NDPs) to deoxynucleotides and are essential for DNA replication and repair. The Escherichia coli RNR, an alpha2beta2 complex, when incubated with 1 equiv of F(2)CDP catalyzes the release of two fluorides and cytosine concomitant with enzyme inactivation. In the presence of reductant (thioredoxin/thioredoxin reductase/NADPH or DTT), the enzyme inactivation results from its covalent labeling of alpha with the sugar of F(2)CDP (one label/alpha2beta2). SDS-PAGE analysis of the inactivated RNR without boiling of the sample reveals that alpha migrates as an 87 and 110 kDa protein in a ratio of 0.6:0.4. When the reductant is omitted, RNR is inactivated by loss of the essential tyrosyl radical and formation of a new radical. Inactivation studies with C225S-alpha in the presence or absence of reductants, reveal it behaves like wt-RNR in the absence of reductant. Inactivated C225S-alpha migrates as an 87 kDa protein and is not covalently modified. C225 is one of the cysteines in RNR's active site that supplies reducing equivalents to make dNDPs. To identify the new radical formed, [1'-(2)H]-F(2)CDP was studied with wt- and C225S-RNR by 9 and 140 GHz EPR spectroscopy. These studies revealed that the new radical is a nucleotide derived with g values of g(x) 2.00738, g(y) 2.00592, and g(z) 2.00230 and with altered hyperfine interactions (apparent triplet collapsed to a doublet) relative to [1'-(1)H]-F(2)CDP. The EPR features are very similar to those we recently reported for the nucleotide radical generated with CDP and E441Q-RNR.