The presence of a cytosine deaminase, which shows no activity on creatinine, was first noted in Alcaligenes denitrificans ssp. denitrificans J9 and Arthrobacter sp. J11. DEAE-Sephacel column chromatography of cellfree extracts of these microorganisms gave a single peak for cytosine deaminase activity, and no fraction contained creatinine deiminase activity. The DEAE-Sephacel active fractions from A. deinitrificans ssp. denitrificans J9 were resolved into two fractions showing cytosine deaminase activity upon Sephacryl S-200 column chromatography; the larger enzyme was termed cytosine deaminase I and the smaller one cytosine deaminase II. Both cytosine deaminases did not deaminate creatinine but did deaminate 5-fluorocytosine rapidly. The molecular masses of cytosine deaminases I and II were estimated to be 200 and 37 kDa, respectively.
