Molecular and Cellular Biochemistry76:27-34 (1987) © Martinus Nijhoff Publishers, Boston - Printed in the Netherlands 27 Deoxycytidylate deaminase activity in non-stimulated and phytohemagglutinin-stimulated human lymphocytes, and in leukemic cells Gerda Tyrsted 1, Pao-chang Chao and Birgitte Munch-Petersen Department of Biochemistry Q University of Copenhagen, Blegdamsvej 3, 2200 Copenhagen N, Denmark (a address for offprints) Received 7 October 1986 Key words." deoxycytidylate deaminase, lymphocytes, human leukemia Abstract Deoxycytidylate deaminase isolated from normal human lymphocytes and from mononuclear leucocytes from patients with acute lymphoblastic leukemia, chronic lymphocytic leukemia and acute monocytic leuke- mia has been characterized in regard to the substrate, dAMP and the allosteric regulators dCTP and dTTP. The enzymes exhibited sigmoidal initial velocity versus dCMP concentration whereas in the presence of the activator, dCTP, Michaelis-Menten kinetics were obtained. At saturating substrate concentrations dTTP acted as an allosteric inhibitor of the enzyme isolated from non-stimulated as well as from stimulated lymphocytes. However, the enzymes isolated from the leukemic ceils had lost the allosteric regulation by dTTP. At low substrate concentrations the competitive inhibitor, dAMP, activated all the enzymes. This activation was abolished in the presence of dCTP which indicates that dAMP might be involved in the regulation of dCMP deaminase activity and thus influence the dCTP and dTTP pools under physiological conditions. Abbreviations: dCMP deaminase, deoxycytidylate deaminase; PHA, Phytohemaggiutinin; ALL, acute lym- phoblastic leukemia; CLL, chronic lymphocytic leukemia; AMOL, acute monocytic leukemia; WBC, white blood cells. Introduction Deoxycytidylate deaminase (dCMP aminohydro- lase EC 3.5.4.12), an allosteric enzyme, has been purified from several sources and characterized [1, 2, 3, 4]. Both dTTP and dCTP serve as allosteric effectors in the presence of a divalent cation such as Mg 2+. Thus, the enzyme is inhibited by dTTP and activated by dCTP [5, 6]. The enzyme activity increases in proliferating cells [7, 8, 9] and it is pos- sible that the enzyme plays an important role in the maintenance of the pools of pyrimidine de- oxyribonucleoside triphosphates, dC-I'P and DTTP [10, 11, 12]. Both deoxyribonucleotides are supplied by the de novo pathway and the salvage pathway, and additionally the dTTP pool is sustained by the dCMP deaminase reaction through the irreversible deamination of dCMP to dUMP which is a precur- sor for the synthesis of dTMP through the reaction catalyzed by thymidylate synthetase (EC 2.1.1.45). In addition to its physiological role there are indica- tions in the literature that dCMP deaminase may be involved directly or indirectly in the metabolization of the pyrimidine antimetabolites used in cancer