Inorganic tripolyphosphate (PPP i) as a phosphate donor for human deoxyribonucleoside kinases

Abstract

Inorganic tripolyphosphate (PPP i) and pyrophosphate (PP i) were examined as potential phosphate donors for human deoxynucleoside kinase (dCK), deoxyguanosine kinase (dGK), cytosolic thymidine kinase (TK1), mitochondrial TK2, and the deoxynucleoside kinase (dNK) from Drosophila melanogaster. PPP i proved to be a good phosphate donor for dGK, as well as for dCK with dCyd, but not dAdo, as acceptor substrate, illustrating also the dependence of donor properties on acceptor. Products of phosphorylation were shown to be 5 ′-phosphates. In striking contrast to ATP, the phosphorylation reaction follows strict Michaelis–Menten kinetics, with K m values of 74 and 92 μM for dCK and dGK, respectively, and V max values 40–50% that for ATP. With the other three enzymes, as well as for dCK with dAdo as acceptor, no, or only low levels (⩽1% of that for ATP) of activity were observed. PP i was inactive (<0.1%) as a phosphate donor with all enzymes, but was a competitive inhibitor vs ATP, as was PPP i in systems with no or low donor activity. This is the first report on inorganic tripolyphosphate as a phosphate donor for nucleoside kinases, in particular human deoxyribonucleoside kinases.