Alkylated cytosine nucleosides: substrate and inhibitor properties in enzymatic deamination

Cytosine nucleoside deaminase (EC 3.5.4.5) from Salmonella typhimurium LT2 catalyses the deamination of ribo-, deoxyribo- and arabinosyl nucleosides of cytosine alkylated at the C-5, but not at the N3 or exocyclic N4, of the pyrimidine ring. The enzyme was inert towards analogues etherified at the 3'-OH and 5'-OH of the sugar ring; it was active against the 2'-O-methyl derivative of cytidine, but not arabinosycytosine. The N4-and 5'-O-alkyl non-substrate analogues competitively inhibited deamination of deoxycytidine and arabinosylcytosine, the most inhibitory being 5'-O-methylarabinosylcytosine. The alpha anomer of 5'-ethyldeoxycytidine, the 2,2'-anhydro derivative of cytidine, and the 3'-O-alkyl derivatives were neither substrates nor inhibitors. The presence of cytidine deaminase was demonstrated in both granulocytes and lymphocytes from human peripheral blood. The specificity of this enzyme differed significantly from that of the bacterial enzyme, a finding of some relevance in relation to the frequently encountered intracellular deamination of therapeutically active arabinosylcytosine to the inactive arabinosyluracil.