Herpes simplex type 1 ribonucleotide reductase. Mechanism studies with inhibitors

Several known inhibitors of mammalian ribonucleotide reductase were studied for their interactions with herpes simplex virus type 1 (HSV-1) ribonucleotide reductase. MAIQ (4-methyl-5-amino-1-formylisoquinoline thiosemicarbazone) produced apparent inactivation of HSV-1 ribonucleotide reductase. Only catalytically cycling, not resting, enzyme could be inactivated. Double reciprocal replots of the rates of inactivation versus the concentration of MAIQ indicated that a reversible complex with the enzyme was formed prior to inactivation. In the presence of 10 microM CDP, the maximum rate of inactivation was 20 per h (t1/2 = 3 min). The half-maximum rate was achieved at about 15 microM MAIQ. INOX (periodate-oxidized inosine) also appeared to inactivate HSV-1 ribonucleotide reductase. In contrast to MAIQ, it readily inactivated resting as well as cycling enzyme. CDP retarded the rates of inactivation by INOX. An initial reversible complex between INOX and enzyme was not detectable under the conditions used. IMPY (2,3-dihydro-1H-pyrazolo(2,3-a)imidazole) and guanazole (3,5-diamino-1,2,4-triazole) produced reversible inhibition. Although the data with both inhibitors were most consistent with the noncompetitive inhibition model (versus CDP), the data with guanazole were also marginally consistent with the uncompetitive model.