Cloning of human adenosine kinase cDNA: sequence similarity to microbial ribokinases and fructokinases.

Adenosine kinase catalyzes the phosphorylation of adenosine to AMP and hence is a potentially important regulator of extracellular adenosine concentrations. Despite extensive characterization of the kinetic properties of the enzyme, its primary structure has never been elucidated. Full-length cDNA clones encoding catalytically active adenosine kinase were obtained from lymphocyte, placental, and liver cDNA libraries. Corresponding mRNA species of 1.3 and 1.8 kb were noted on Northern blots of all tissues examined and were attributable to alternative polyadenylylation sites at the 3' end of the gene. The encoding protein consists of 345 amino acids with a calculated molecular size of 38.7 kDa and does not contain any sequence similarities to other well-characterized mammalian nucleoside kinases, setting it apart from this family of structurally and functionally related proteins. In contrast, two regions were identified with significant sequence identity to microbial ribokinase and fructokinases and a bacterial inosine/guanosine kinase. Thus, adenosine kinase is a structurally distinct mammalian nucleoside kinase that appears to be akin to sugar kinases of microbial origin.

beta-Hydroxybutyrate decreases adenosine triphosphate degradation products in human subjects.

Many disease states decrease intracellular adenosine triphosphate (ATP) levels and elevate body fluid purine levels. The use of specific metabolic substrates may reverse this process. This study was designed to test the hypothesis that beta-hydroxybutyrate, a substrate for ATP synthesis, decreases body fluid purine levels during interventions that induce ATP degradation. Decreases in these purine levels are metabolic markers for diminished ATP degradation. Two human models for stimulating ATP degradation were used to test the hypothesis. Rapid fructose infusion causes acute degradation of hepatic ATP, and ischemic exercise stimulates ATP consumption in skeletal muscle. The activity of beta-hydroxybutyrate was used in combination with phosphate, another important substrate for ATP synthesis. The studies were performed during a low-phosphate state in 10 normal subjects and during a high-phosphate state in 7 normal subjects. Metabolic variables, such as serum or urinary phosphate level, blood beta-hydroxybutyrate level, blood acetoacetate level, plasma or urinary purine level, blood lactate level, and blood ammonia level, were monitored during the study. After ischemic exercise of the forearm muscle, beta-hydroxybutyrate decreased the level of plasma total purines, blood lactate, and blood ammonia during the low-phosphate state but not during the high-phosphate state. During fructose-induced hepatic ATP breakdown, beta-hydroxybutyrate decreased late phase plasma purine increases under low-phosphate conditions only and decreased urinary total and radiolabeled purine elevations under both phosphate conditions. These data indicate that the infusion of beta-hydroxybutyrate may alter the balance from ATP degradation toward ATP resynthesis in muscle and liver by providing an immediate source of fuel and reducing equivalents under under specific metabolic conditions. This activity in combination with other metabolic interventions may have therapeutic value by restoring ATP pools in ATP-depleted tissues.

Altered properties of human T-lymphoblast soluble low Km 5'-nucleotidase: comparison with B-lymphoblast enzyme.

Soluble low Km 5'-nucleotidases have been purified from human cultured T- and B-lymphoblasts to compare their properties and to examine the mechanism of different rates of nucleotide dephosphorylation. The enzyme from B-lymphoblasts (MGL-8) was 4385-fold purified with a specific activity of 114 mumol/min/mg, while the enzyme from T-lymphoblasts (CEM, MOLT-4) was 4355-fold purified with a specific activity of 35 mumol/min/mg. The activity of both enzymes have an absolute requirement for Mg++. The B-cell enzyme has maximum activity with Mg2+ > Mn2+ > Co2+, while the T-cell enzyme had maximum activity with Co2+ > Mn2+ > Mg2+. The optimum activity was at pH 7.4-9.0 for the B-cell enzyme and pH 9.0 for the T-cell enzyme. Substrate specificity was the same for both enzymes with the following relative Vmax values: CMP > UMP > dUMP > dCMP > dAMP > IMP > GMP > dIMP > dGMP. The Km values for AMP and IMP were 12 and 25 microM for the B-cell enzyme, and 7.0 and 12 microM for the T-cell enzyme. ATP and ADP are competitive inhibitors of these enzymes with apparent Ki values of 100 and 20 microM for the B-cell enzyme, and 44 microM and 8 microM for the T-cell enzyme, respectively. The apparent molecular mass by gel filtration column chromatography is 145 kD for the B-cell enzyme and 72 kDa for the T-cell enzyme. The subunit molecular masses by Western blots are 69.2 kD for both enzymes. These properties suggest that the B-lymphoblast enzyme is identical or similar to the enzyme from human placenta. However, the T-cell enzyme has some different properties. We conclude that these differences plus a lower content of low Km 5'-nucleotidase in T-cells may account for the decreased ability of T-lymphoblasts to dephosphorylate nucleotides and may contribute to the selective cytotoxicity of deoxyribonucleosides for T-lymphoblasts as compared to B-lymphoblasts.

Adenotin and adenotin-like proteins coexist with adenosine receptors in mammalian tissues.

The relationship of adenotin, a low-affinity adenosine-binding protein, to adenosine receptors was examined in two human tissues and two mammalian cultured cell lines. An adenosine A2 receptor exists in the membranes from platelets, PC-12 cells, and JAR cells as shown by a stimulation of adenylate cyclase related to 5'-N-ethylcarboxamidoadenosine (NECA) or a NECA-related increase in intracellular cAMP levels. In contrast, binding studies with tritiated NECA revealed typical adenotin-like low-affinity binding sites on the membranes from the sources studied with agonist potencies as follows: NECA greater than 2-chloroadenosine greater than R-PIA. No evidence was found of coupling to a guanine nucleotide regulatory protein. Solubilization of platelet and placental membranes and precipitation with polyethylene glycol separated adenotin or the adenotin-like protein from a second adenosine binding site in each tissue. The pharmacologic properties of the precipitated binding sites were compatible with an adenosine A2 receptor in platelets and an adenosine A1 receptor in placenta. Our observations indicate that adenotin-like proteins exist outside the placenta. In addition, adenotin and adenotin-like proteins coexist with the adenosine A1 or A2 receptor in a number of cells and tissues and do not couple to a guanine nucleotide regulatory protein and stimulate adenylate cyclase. Therefore, adenotin is pharmacologically distinct from adenosine receptors, and its function remains to be discovered.

Cloning and expression of human deoxycytidine kinase cDNA.

Deoxycytidine (dCyd) kinase is required for the phosphorylation of several deoxyribonucleosides and certain nucleoside analogs widely employed as antiviral and chemotherapeutic agents. Detailed analysis of this enzyme has been limited, however, by its low abundance and instability. Using oligonucleotides based on primary amino acid sequence derived from purified dCyd kinase, we have screened T-lymphoblast cDNA libraries and identified a cDNA sequence that encodes a 30.5-kDa protein corresponding to the subunit molecular mass of the purified protein. Expression of the cDNA in Escherichia coli results in a 40-fold increase in dCyd kinase activity over control levels. In dCyd kinase-deficient murine L cells, transfection with dCyd kinase cDNA in a mammalian expression vector produces a 400-fold increase over control in dCyd phosphorylating activity. The expressed enzyme has an apparent Km of 1.0 microM for dCyd and is also capable of phosphorylating dAdo and dGuo. Northern blot analysis reveals a single 2.8-kilobase mRNA expressed in T lymphoblasts at 5- to 10-fold higher levels than in B lymphoblasts, and decreased dCyd kinase mRNA levels are present in T-lymphoblast cell lines resistant to arabinofuranosylcytosine and dideoxycytidine. These findings document that this cDNA encodes the T-lymphoblast dCyd kinase responsible for the phosphorylation of dAdo and dGuo as well as dCyd and arabinofuranosylcytosine.

Properties of adenotin reconstituted into phospholipid vesicles.

Adenotin is a low affinity adenosine binding protein that has amino terminal homology with mammalian and avian stress proteins. Human placental adenotin was solubilized and reconstituted into phospholipid vesicles with an overall yield of 30%. The properties of adenotin in vesicles were similar to the native membranes as follows: association has a Kobs of 0.61 +/- 0.03 minute-1; equilibrium is reached in approximately 15 minutes; and the first order dissociation constant is 5.0 +/- 0.3 minute-1. Displacement analysis reveals an agonist potency order and Ki values as follows: N-ethylcarboxamidoadenosine, 0.35 microM; 2-chloroadenosine, 1.5 microM; R-phenylisopropyladenosine, greater than 1000 microM. The addition of 100 microM 5'-guanylylimidodiphosphate did not decrease binding of 5'-N-ethylcarboxamidoadenosine (NECA) at 37 degrees C or 4 degrees C but did decrease the IC50 for PC12 and JAR cell membrane agonist binding from 9.9 to 3.3 microM and increase the binding to 150-211% of the control value at 37 degrees C. The latter studies at 37 degrees C showed high variability. Using binding sites reconstituted into vesicles and gel filtration chromatography and agonist related guanine nucleotide release, the authors investigated whether these changes were related to an interaction between adenotin and a guanine nucleotide regulatory protein. No evidence for such an interaction was found. These data suggest that adenotin retains its binding properties when reconstituted into phospholipid vesicles. The function of this low affinity adenosine binding site remains to be discovered. However, the reconstitution of adenotin into phospholipid vesicles provides a method to study its function.

Soluble and membrane-associated human low-affinity adenosine binding protein (adenotin): properties and homology with mammalian and avian stress proteins.

A low-affinity adenosine binding protein has recently been distinguished from the adenosine A2 receptor and purified from human placental membranes. Soluble human placental extracts contain an adenosine binding activity that has properties similar to those of the membrane low-affinity adenosine binding protein. The binding protein was purified from soluble human placental extracts 134-fold to 89% purity with a Bmax of 2.5 nmol/mg. It comprises 0.7-0.9% of the soluble protein. The major purified soluble protein has a subunit molecular mass of 98 kDa and a Stokes radius identical with that of the membrane-bound adenosine binding protein. Competition analysis of the soluble protein revealed similar affinities and an identical potency order for displacement of 5'-(N-ethylcarbamoyl)[2,8-3H]adenosine ([3H]NECA) as follows: NECA greater than 2-chloroadenosine greater than adenosine greater than (R)-N6-(2-phenylisopropyl)adenosine. The soluble binding protein was more acidic than the membrane binding protein as revealed by a comparison of the elution properties during ion exchange chromatography. A second form of soluble adenosine binding activity comprised 17% of the major form and had a charge similar to that of the membrane binding protein, a smaller Stokes radius, and a subunit molecular mass of 74 kDa. Carbohydrate composition analysis revealed that the major soluble form has 4.3% carbohydrate by weight as compared to the membrane-associated form, which has 5.5% carbohydrate by weight.(ABSTRACT TRUNCATED AT 250 WORDS)

Partial separation of platelet and placental adenosine receptors from adenosine A2-like binding protein.

The ubiquitous adenosine A2-like binding protein obscures the binding properties of adenosine receptors assayed with 5'-N-[3H]ethylcarboxamidoadenosine [( 3H]NECA). To solve this problem, we developed a rapid and simple method to separate adenosine receptors from the adenosine A2-like binding protein. Human platelet and placental membranes were solubilized with 1% 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate. The soluble platelet extract was precipitated with polyethylene glycol and the fraction enriched in adenosine receptors was isolated from the precipitate by differential centrifugation. The adenosine A2-like binding protein was removed from the soluble placental extract with hydroxylapatite and adenosine receptors were precipitated with polyethylene glycol. The specificity of the [3H]NECA binding is typical of an adenosine A2 receptor for platelets and an adenosine A1 receptor for placenta. This method leads to enrichment of adenosine A2 receptors for platelets and adenosine A1 receptors for placenta. This provides a useful preparation technique for pharmacologic studies of adenosine receptors.

Metabolic alterations in burn patients: detection of adenosine triphosphate degradation products and lipid peroxides.

Seven patients admitted to the University of Michigan Burn Center with greater than 20 per cent total body surface area burns were studied for evidence of oxygen radical production, as demonstrated by serum lipid peroxides, and adenosine triphosphate (ATP) degradation, based upon ATP degradation products in blood (serum purines) and urine (urine purines and urine uric acid). Lipid peroxides (conjugated dienes) were elevated beginning on day 1 postburn and remained elevated up to day 5, the duration of the study. ATP degradation products were elevated during the initial 24 h postburn but rapidly fell to normal levels. This study provides evidence of biochemical alterations in thermally injured patients similar to observations in animal models which demonstrate that ATP degradation and the production of oxygen radicals are part of the initial response to thermal trauma.

Purification and characterization of the adenosine A2-like binding site from human placental membrane.

We have purified and characterized the adenosine A2-like binding site from human placental membranes. 5'-N-Ethylcarboxamido[2,8-3H]adenosine ([3H]NECA) binds to this site, with a Kd of 240 nM and a Bmax of 13.0 pmol/mg in human placental membranes. The adenosine A2-like binding site was purified after extraction from placental membranes with 0.1% 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonic acid. The purification included ammonium sulfate precipitation and concanavalin A, DEAE-Sephadex, and Sepharose 6B gel filtration chromatographies. The protein was purified 127-fold to homogeneity, with a final specific activity of 1.5-1.9 nmol/mg of protein and a 5.5-8.1% yield of binding activity from the membranes. The purified protein had similar binding properties and an identical potency order for displacement of [3H] NECA by adenosine analogs as the initial membranes. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of purified protein revealed a single band at 98 kDa which coeluted with [3H]NECA binding activity during Sepharose 6B gel filtration chromatography. In 0.1% Triton X-100, the binding complex has a Stokes radius of 70 A, a sedimentation coefficient of 6.9 S, and a partial specific volume of 0.698 ml/g. The detergent-protein complex has a calculated molecular mass of 230 kDa. The estimated frictional ratio is 1.5. The native binding complex appears to consist of a dimer of identical subunits. The function of this ubiquitous protein remains unclear.

Adenosine triphosphate turnover in humans. Decreased degradation during relative hyperphosphatemia.

The regulation of ATP metabolism by inorganic phosphate (Pi) was examined in five normal volunteers through measurements of ATP degradation during relative Pi depletion and repletion states. Relative Pi depletion was achieved through dietary restriction and phosphate binders, whereas a Pi-repleted state was produced by oral Pi supplementation. ATP was radioactively labeled by the infusion of [8(14)C]adenine. Fructose infusion was used to produce rapid ATP degradation during Pi depletion and repletion states. Baseline measurements indicated a significant decrease of Pi levels during phosphate depletion and no change in serum or urinary purines. Serum values of Pi declined 20 to 26% within 15 min after fructose infusion in all states. Urine measurements of ATP degradation products showed an eightfold increase within 15 min after fructose infusion in both Pi-depleted and -supplemented states. Urinary radioactive ATP degradation products were fourfold higher and urinary purine specific activity was more than threefold higher during Pi depletion as compared with Pi repletion. Our data indicate that there is decreased ATP degradation to purine end products during a relative phosphate repletion state as compared to a relative phosphate depletion state. These data show that ATP metabolism can be altered through manipulation of the relative Pi state in humans.

Cyclosporine-induced hyperuricemia and gout.

To evaluate the frequency and the pathogenesis of hyperuricemia and gout during cyclosporine therapy, we studied renal-transplant recipients who were treated with either cyclosporine and prednisone (n = 129) or azathioprine and prednisone (n = 168). Among the patients with stable allograft function and serum creatinine concentrations below 265 mumol per liter, hyperuricemia was more common in the cyclosporine group than in the azathioprine group (84 percent vs. 30 percent; P = 0.0001). Gout developed in nine patients (7 percent) in the cyclosporine group, but no episodes occurred in the azathioprine group. Serum urate levels became elevated in 90 percent of the patients in the cyclosporine group who were treated with diuretics, as compared with 60 percent of those not treated with diuretics (P = 0.001); in the azathioprine group, the corresponding values were 47 percent and 15 percent (P = 0.0001). Serum urate levels did not correlate with trough blood cyclosporine levels in a selected subgroup (n = 40) of patients from the cyclosporine group, who were studied from 4 to 96 weeks after transplantation. Detailed studies of urate metabolism in six cyclosporine-treated patients revealed normal turnover rates for urate and decreases in creatinine and urate clearance, as compared with seven control subjects. We conclude that hyperuricemia is a common complication of cyclosporine therapy and is caused by decreased renal urate clearance. Gouty arthritis is the cause of considerable morbidity among renal-transplant recipients who receive cyclosporine.

Kinetic properties and inhibition of human T lymphoblast deoxycytidine kinase.

The kinetic properties of 50,000-fold purified cultured human T lymphoblast (MOLT-4) deoxycytidine kinase were examined. The reaction velocity had an absolute requirement for magnesium. Maximal activity was observed at pH 6.5-7.0 with Mg:ATP for 1:1. High concentrations of free Mg2+ or free ATP were inhibitory. Double reciprocal plots of initial velocity studies yielded intersecting lines for both deoxycytidine and MgATP2-. dCMP was a competitive inhibitor with respect to deoxycytidine and ATP. ADP was a competitive inhibitor with respect to ATP and a mixed inhibitor with respect to deoxycytidine. dCTP, an important end product, is a very potent inhibitor and was a competitive inhibitor with respect to deoxycytidine and a non-competitive inhibitor with respect to ATP. TTP reversed dCTP inhibition. The data suggest that (a) MgATP2- is the true substrate of deoxycytidine kinase; (b) the kinetic mechanism of deoxycytidine kinase is consistent with rapid equilibrium random Bi Bi; (c) deoxycytidine kinase may be regulated by its product ADP and its end product dCTP as well as the availability of deoxycytidine. While many different nucleotides potently inhibit deoxycytidine kinase, their low intracellular concentrations make their regulatory role less important.

Adenine nucleotide turnover in hypoxanthine-guanine phosphoribosyl-transferase deficiency: evidence for an increased contribution of purine biosynthesis de novo.

This study examined whether increased purine biosynthesis de novo in HGPRT deficiency contributes to adenine nucleotide formation compared with normal subjects. Four HGPRT deficient patients and four normal subjects received intravenously 10 to 25 microCi of [8-14C]adenine to radiolabel the adenine nucleotide pool followed five days later by a rapid infusion of fructose to stimulate purine nucleotide degradation. Fructose infusion increased urinary radioactivity in the enzyme-deficient patients to 141% +/- 13% (mean +/- SEM) of the baseline values compared with 1,067% +/- 102% in normal subjects (P less than .01). The absolute mean increase in total urinary purines in the patients was 17.96 +/- 3.36 and 10.38 +/- 3.80 mmol/g creatinine in controls (P less than .05). The apparent specific radioactivity of urinary purines increased in the control group from a mean of 1.29 X 10(5) to 3.64 X 10(5) cpm/mmol of purines (P less than .02) but decreased in the enzyme-deficient subjects from a mean of 1.66 X 10(5) to 1.38 X 10(5) cpm/mmol. To assess if the decrease in the specific activity of urinary purines was due to an elevated rate of de novo purine synthesis, two HGPRT-deficient patients were treated with allopurinol and adenine followed five days later by a fructose infusion. The administration of adenine increased the specific activity of urinary purines after the infusion of fructose from a mean baseline value of 1.05 X 10(5) to 1.42 X 10(5) cpm/mmol of purines.(ABSTRACT TRUNCATED AT 250 WORDS)

AMP and IMP dephosphorylation by soluble high- and low-Km 5'-nucleotidases.

Three distinct 5'-phosphomonoesterase activities were isolated from soluble fractions of human placenta, cultured human T and B lymphoblasts, and rat liver using 5'-AMP-sepharose 4B affinity chromatography. We define these activities as "low-Km" 5'-nucleotidase, "high-Km" 5'-nucleotidase, and nonspecific phosphatase. High-Km 5'-nucleotidase was eluted with 0.5 M NaCl, low-Km 5'-nucleotidase was eluted with 10 mM ADP, and nonspecific phosphatase was not retained on the column. We have found significant variability in the relative content of high- to low-Km activities in the tissues studied with the ratios ranging from 5.5 to 264. The properties were studied after further purification. The molecular mass of the low-Km enzymes ranged from 72.5 to 209 kDa, optimum pH ranged from 7.4 to 9.0, Km for AMP ranged from 7 to 15 microM, and Km for IMP ranged from 10 to 26 microM. The molecular mass of the high-Km enzymes ranged from 182 to 210 kDa, pH optimum was at 6.5, Km for AMP ranged from 3.0 to 9.4 mM, and the Km for IMP ranged from 0.3 to 0.5 mM. The data indicate that the soluble low- and high-Km 5'-nucleotidase coexist in the mammalian cells and tissues studied. These observations suggest a complex system for the regulation of nucleoside 5'-monophosphate dephosphorylation.

The application of affinity chromatography for the separation of "high Km" and "low Km" 5'-nucleotidase and other AMP metabolizing enzymes.

AMP-sepharose 4B has been widely used as a general ligand affinity chromatography for purification of AMP deaminase, 5'-nucleotidase, adenosine kinase and other adenine nucleotide metabolizing enzymes. Since these enzymes generally differ in their kinetic properties related to the values of Km for AMP and analogous compounds, it was assumed that there may be a specific elution pattern of some of the enzymes which would enable sequential elution from the column during a single run. Using 0.5 M NaCl, 10 mM ATP and 5 mM adenosine as eluting agents, it was possible to separate on AMP-sepharose column AMP deaminase "high Km" and "low Km" 5'-nucleotidase and adenosine kinase. Adenylate kinase, adenosine deaminase and nonspecific phosphatase did not bind to the column. Using human placental extract, AMP deaminase, "high Km" and "low Km" 5'-nucleotidase and adenosine kinase were purified 2.8, 2.9, 105 and 1240 fold, respectively. AMP deaminase and "high Km" 5'-nucleotidase were further separated using phosphocellulose column chromatography and the final purification was 227 and 143 fold, respectively. The specific activities of purified enzyme preparations were 9.1, 1.0, 0.4 and 0.5 mumols/min/mg protein of AMP deaminase, "high Km" 5'-nucleotidase and adenosine kinase, respectively. This approach provides a rapid method for initial purification of these enzymes from crude soluble extracts.

Evidence for "low Km" and "high Km" soluble 5'-nucleotidases in human tissues and rat liver.

Two kinetically distinct purine 5'-phosphomono-esterase activities were isolated from soluble fractions of human placenta, cultured human T- and B-lymphoblasts and rat liver using AMP-sepharose chromatography. We have defined these activities as "high Km" and "low Km" 5'-nucleotidase. The relative content of "high Km" and "low Km" activities in the tissues studied ranged from 2 to 264. The optimum pH of "low Km" 5'-nucleotidases ranged from 7.4 to 9.0, Km for AMP from 7 to 15 uM and for IMP from 10 to 26 uM. ATP and ADP were inhibitors of "low Km" enzymes with the apparent Ki values of 55 to 20 uM and 8 to 20 uM for ATP and ADP, respectively. "High Km" 5'-nucleotidases had an optimum pH at 6.5, Km for IMP of 0.3 to 0.5 mM and Km for AMP of 1.0 to 9.4 mM. "High Km" enzymes were activated by ATP with A0.5 values, of 1.7 to 2.3 mM at 100 microM IMP. The data indicate that soluble "low Km" and "high Km" 5'-nucleotidases coexist in mammalian cells and fulfill different functions. These observations suggest a complex system for the regulation of AMP and IMP dephosphorylation.

Human T-lymphoblast deoxycytidine kinase: purification and properties.

Previous observations present tremendous variations in the properties of deoxycytidine kinase. To clarify the properties and physiologic role of deoxycytidine kinase, we have undertaken its purification. Deoxycytidine kinase was purified from cultured human T-lymphoblasts (MOLT-4) to 90% purity with an estimated specific activity of 8 mumol min-1 (mg of protein)-1. The purification procedure included ammonium sulfate precipitation, Superose-12 HPLC gel filtration chromatography, DE-52 ion-exchange chromatography, AMP-Sepharose 4B affinity chromatography, and dCTP-Sepharose-4B affinity chromatography. Deoxyguanosine, deoxyadenosine, and cytidine phosphorylating activities copurified with deoxycytidine kinase to final specific activities of 7.2, 13.5, and 4 mumol min-1 (mg of protein)-1, respectively. The enzyme is very unstable at low protein concentration and is stabilized by storage at -85 degrees C with 1 mg/mL bovine serum albumin, 20% glycerol (v/v), 200 mM potassium chloride, and 25 mM dithiothreitol. The molecular weight was 60,000, and the Stokes radius was 32 A by gel filtration chromatography. The subunit molecular weight was 30,500. This enzyme had apparent Km values of 1.5, 430, 500, 450, and 40 microM for deoxycytidine, deoxyguanosine, deoxyadenosine, cytidine, and cytosine arabinoside, respectively. The pH optimum ranged from 6.5 to 9.0. Mg2+ and Mn2+ were the preferred divalent cations. ATP, GTP, dGTP, ITP, dITP, TTP, and XTP were substrates for the enzymes. Our study indicates that deoxycytidine kinase is a dimer with two subunits and has phosphorylating activity for deoxyguanosine, deoxyadenosine, cytidine, and cytosine arabinoside. This highly purified enzyme will facilitate the study of its regulation and phosphorylation of anticancer or antiviral nucleoside analogues.