Cytidine Deaminase from Escherichia coli

Cytidine deaminase (cytidine aminohydrolase, EC 3.5.4.5) has been purified approximately 160-fold from extracts of Escherichia coli B. The enzyme shows constant activity between pH 6 and 11. No significant change in rate is observed when DzO replaces water as the solvent. In addition to the deamination of cytidine, the purified deaminase catalyzes slow hydrolysis of N4-methylcytidine. The enzyme is competitively inhibited by 3,4,5,6-tetrahydrouridine, previously shown to inhibit the deamination of 1-fi-D-arabinofuranosylcytosine by preparations of human liver. The bacterial enzyme exhibits an affinity for this inhibitor exceeding by more than four orders of magnitude its combined affinity for the products uridine and ammonia.