Gammaherpesviruses encode functional dihydrofolate reductase activity.

We overexpressed and purified from Escherichia coli the dihydrofolate reductase (DHFR) of the gammaherpesviruses human herpesvirus 8 (HHV-8), herpesvirus saimiri (HVS), and rhesus rhadinovirus (RRV). All three enzymes proved catalytically active. The K(m) value of HHV-8 DHFR for dihydrofolate (DHF) was 2.02+/-0.44 microM, that of HVS DHFR was 4.31+/-0.56 microM, and that of RRV DHFR is 7.09+/-0.11 microM. These values are approximately 5-15-fold higher than the K(m) value reported for the human DHFR. The K(m) value of HHV-8 DHFR for NADPH was 1.31+/-0.23 microM, that of HVS DHFR was 3.78+/-0.61 microM, and that of RRV DHFR was 7.47+/-0.59 microM. These values are similar or slightly higher than the corresponding K(m) value of the human enzyme. Methotrexate, aminopterin, trimethoprim, pyrimethamine, and N(alpha)-(4-amino-4-deoxypteroyl)-N(delta)-hemiphthaloyl-L-ornithine (PT523), all well-known folate antagonists, inhibited the DHFR activity of the three gammaherpesviruses competitively with respect to DHF but proved markedly less inhibitory to the viral than towards the human enzyme.