As for some reactions using NDPs as substrates and NTPs as phosphate donor NMP formation was detected, we validated whether NMP formation is the result of the reverse NMPK reaction or based on basal NDP cleaving activity of the enzyme preparation. Reactions were performed with NDP as substrate, but without ATP. Indeed, with some substrates, NMPKs (except for UMPK) were able to catalyze the reaction 2 NDP ↔ NMP + NTP. Accepted substrates correlated well with the NMP preference of the enzymes (Figure ). As an example, Tm(d)CMPK converted (d)CDP and UDP to the corresponding NMP and NTP.