Factors affecting deoxycytidylate deaminase activity in some transplantable rat hepatomas

Dunning hepatoma has a low activity of deoxycytidylate deaminase, comparable to that of normal adult rat liver. This activity seems inconsistent with the rapid proliferation rate of the tumor. Factors which might affect the activity of deoxycytidylate deaminase in the Dunning hepatoma have been examined in it and compared to the Novikoff hepatoma which has high activity of this enzyme. The low activity in Dunning hepatoma does not appear to be the result of any inhibition or, possibly, proteolytic enzyme as judged by mixing experiments, nor does it appear to be due to in vivo differences in nucleotide concentrations especially deoxycytidine 5'-monophosphate, deoxycytidine 5'-triphosphate, or deoxyguanosine 5'-monophosphate which might either help stabilize the enzyme, allosterically increase its activity, or inhibit it. The Dunning hepatoma does not convert cytosine deoxyriboside to uridine deoxyriboside at a significant rate, and the formation of uridine deoxyriboside from deoxyuridine monophosphate is 1% or less during a 30-min incubation of high-speed supernatant fraction from the tumor in either the presence or absence of fluoride. It is concluded that the Dunning hepatoma probably has intrinsically low deoxycytidylate deaminase activity.