Characterization of normal and mutant adenosine deaminase messenger RNAs by translation and hybridization to a cDNA probe

Summary Using both in vitro translation and hybridization to an adenosine deaminase (ADA) cDNA probe, ADA mRNA has been characterized in B lymphoblast lines established from seven ADA-deficient children, two parents of an ADA-deficient child, and three normal people. All ADA-deficient lines except GM-2825A, including those with less than 1% of normal catalytic activity, had normal or greater amounts of hybridizable, 1.6 kilobase in size, ADA mRNA. Immunoreactive ADA protein of normal size was produced by in vitro translation of the mRNAs. Deficiency of ADA activity in these lines appears secondary to synthesis of structurally altered proteins rather than to a quantitative deficiency in ADA mRNA. The GM-2825A line contains electrophoretically abnormal species of RNA which hybridize to the cDNA probe. Deficiency of ADA activity in this line appears at least in part secondary to a structural defect in the ADA mRNA or its precursors.     

Heterogeneity for adenosine deaminase deficiency: Expression of the enzyme in cultured skin fibroblasts and amniotic fluid cells.

Adenosine deaminase (ADA) could be quantitated and the isozyme pattern characterized in cultured amniotic fluid cells. In 20 amniotic fluid cell cultures the mean specific activity was 14.3 U/g protein +/- 6.7 (SD) and compared favorably with values of 14.6 U/g protein +/- 6.8 (SD) observed in 26 cultures of skin fibroblasts. In cultures of skin fibroblasts established from two obligate heterozygotes for ADA deficiency, the specific activity of ADA was 7.0 and 7.7 U/g protein. The ADA isozyme pattern that existed in cultures of amniotic fluid cells was the same as that observed in cultured skin fibroblasts. This identification of the same apparent enzyme may permit the prenatal diagnosis of that form of combined immunodeficiency disease caused by ADA deficiency. Residual enzyme activity of less than 1% and 10% of the mean of normal fibroblasts could be measured in cultured fibroblasts from two unrelated children with ADA deficiency and combined immunodeficiency disease. The tissue-specific enzyme from cultured skin fibroblasts from the child with 10% residual activity had a faster electrophoretic mobility and greater heat stability than normal ADA. This enzymatic evidence indicates that at least two mutant alleles exist at the locus for ADA which predispose to combined immunodeficiency disease when present in the homozygous state.     

Metabolism of purine nucleosides and phosphoribosylpyrophosphate in thymocytes and splenocytes of various mammalian species

1. Activities of ADA, PNP and AK were measured in splenocytes and thymocytes of newborn children, young horses, pigs, sheep, rats and mice and compared with the activities previously found in peripheral lymphocytes. 2. With all species, except horse, the activity of ADA (per 10(6) cells) was higher in thymocytes than in lymphocytes. Activity of ADA was highest in splenocytes of pig and sheep. Activity of ADA was lowest in all lymphoid cells of the horse and only about 10% of the activity in human splenocytes and lymphocytes. 3. With all species, except horse, the activity of PNP was lower in thymocytes than in lymphocytes. Activity of PNP was highest in human lymphocytes and lowest in ovine thymocytes. 4. Activity of AK is comparable in thymocytes of all species and always lower than the ADA activity. In splenocytes of man, horse and pig the activity of AK is comparable to that in thymocytes. 5. Activity of deoxyguanosine kinase was lowest in rat thymocytes and highest in those of man. 6. When enzyme activities are expressed per milligram of protein, the differences between thymocytes and lymphocytes are less pronounced. 7. Activity of PRPP synthetase per 10(6) cells was comparable in thymocytes, splenocytes and lymphocytes of the same species and between the various species. 8. The concentration of PRPP was lowest in ovine thymocytes and higher in splenocytes than in thymocytes of the same species, except man.     

Rat Brain Adenosine Deaminase After 2''-Deoxycoformycin Administration: Biochemical Properties and Evidence for Reduced Enzyme Levels Detected by 2''-[3H]Deoxycoformycin Ligand Binding

Near total inhibition of brain adenosine deaminase (ADA) activity in rats injected with the potent ADA inhibitor 2'-deoxycoformycin (DCF) was previously shown to reduce enzyme activity for up to 50 days during which time the enzyme exhibited reduced sensitivity to in vivo inhibition by DCF. Here, we investigated the biochemical properties of ADA and the basis for its reduced activity after DCF treatment. It was found that much higher doses of DCF were required to inhibit ADA in DCF-treated compared with drug-naive animals. Fourteen days after DCF administration, reduced ADA activity in brain homogenates was due to a decrease in Vmax, rather than to an altered Km of ADA for adenosine. DCF treatment had no effect on Ki values for erythro-9-(2-hydroxy-3-nonyl)adenine inhibition of ADA. The IC50 value for DCF inhibition of ADA in hypothalamus was unchanged. However, the Ki for DCF inhibition of ADA in whole brain increased by fivefold. Sucrose gradient analysis of brain ADA revealed only one corresponding peak of activity and [3H]DCF-labeled ADA in DCF-treated and control rats. A radioligand filtration assay with [3H]DCF was developed to assess the effects of DCF on ADA protein levels. Over a roughly 200-fold range of ADA activities the binding of [3H]DCF was highly correlated with deaminase activity (r = 0.99). In brain tissues taken 8 and 33 days after treatment of rats with DCF, [3H]DCF binding was reduced to 27% and 48% of control levels, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)     

Enzyme activity and distribution of adenosine deaminase types in a population of central Italy

Blood samples from 718 unrelated individuals born in municipalities of the 'Alto Maceratese' region in central Italy were examined for adenosine deaminase (ADA). The allele frequencies for ADA*1 and ADA*2 were 92.48 and 7.45%, respectively. One case of the rare ADA 5-1 phenotype was observed. The observed phenotype distribution agreed well with the one expected assuming a Hardy-Weinberg equilibrium. Enzyme activity was measured in red blood cells from individuals with different phenotypes. The relationships between the enzyme activity of the phenotypes was found to be ADA 1 greater than ADA 2-1 greater than ADA 2 greater than ADA 5-1.     

Molecular basis for paradoxical carriers of adenosine deaminase (ADA) deficiency that show extremely low levels of ADA activity in peripheral blood cells without immunodeficiency

Adenosine deaminase (ADA) deficiency causes an autosomal recessive form of severe combined immunodeficiency and also less severe phenotypes, depending to a large degree on genotype. In general, ADA activity in cells of carriers is approximately half-normal. Unexpectedly, healthy first-degree relatives of two unrelated ADA-deficient severe combined immunodeficient patients (mother and brother in family I; mother in family II) had only 1-2% of normal ADA activity in PBMC, lower than has previously been found in PBMC of healthy individuals with so-called "partial ADA deficiency." The level of deoxyadenosine nucleotides in erythrocytes of these paradoxical carriers was slightly elevated, but much lower than levels found in immunodeficient patients with ADA deficiency. ADA activity in EBV-lymphoblastoid cell lines (LCL) and T cell lines established from these carriers was 10-20% of normal. Each of these carriers possessed two mutated ADA alleles. Expression of cloned mutant ADA cDNAs in an ADA-deletion strain of Escherichia coli indicated that the novel mutations G239S and M310T were responsible for the residual ADA activity. ADA activity in EBV-LCL extracts of the paradoxical carriers was much more labile than ADA from normal EBV-LCL. Immunoblotting suggested that this lability was due to denaturation rather than to degradation of the mutant protein. These results further define the threshold level of ADA activity necessary for sustaining immune function.     

利用复感儿T淋巴细胞作为外源性ADA基因表达效应细胞的实验探讨

对４１例健康儿童和１７例反复呼吸道感染患儿（复咸儿）的外周血淋巴细胞ＡＤＡ活性进行了检测．两例ＡＤＡ活性明显低下的复感儿的外周血Ｔ淋巴细胞被选作实验样品,经体外培养后,采用ｌｉｐｏｆｅｃｔｉｎ介导的的方法导入外源性ＡＤＡ基因．结果表明：体外培养的两例复感儿淋巴细胞中ＡＤＡ活性较基因转移前升高;细胞免疫学特性也在一定程度上获得改善     

腺苷脱氨酶活性对自身免疫性肝病的临床意义研究

摘要 目的：探讨腺苷脱氨酶（ADA）在自身免疫性肝病患者血清中的变化及其临床意义。方法：利用酶法试剂盒检测自身免疫性肝病患者血清中的总ADA（tADA）及其同工酶ADA1和ADA2的活性变化,利用受试者工作曲线（ROC）分析总ADA、ADA1及ADA2活性的诊断价值。利用Spearman相关性分析自身免疫性肝病患者各指标之间的相关性。结果：与对照组相比,tADA和ADA2活性在自身免疫性肝病患者血清中均极显著升高（P<0.001）,ADA1活性有一定程度升高（P=0.035）。不同自身免疫性肝病亚型患者之间的血清tADA、ADA1及ADA2活性均无显著差异。ROC分析显示,血清tADA和ADA2活性具有诊断价值,ADA1活性无诊断价值。tADA活性截断值取13.5 U/L时,诊断特异性为93.3%,敏感性为81.2%。血清ADA2活性截断值取9.5 U/L时,诊断特异性为85.0%,敏感性为83.3%。而ADA1无显著的诊断价值,ADA活性与白蛋白球蛋白比值呈显著负相关（r=-0.41,P=0.004）,与球蛋白水平具有一定程度的正相关（r=0.34,P=0.018）,与ALT呈弱正相关（r=0.29,P=0.042）,与其他指标的相关性均无统计学显著性。结论：血清tADA及ADA2活性在自身免疫性肝病患者血清中显著升高,并且具有一定的诊断价值。     

腺苷脱氨酶及同工酶对自身免疫病诊断和病情监测的作用

摘要 目的：检测腺苷脱氨酶(ADA)及其同工酶在自身免疫病患者血清中的变化,探讨其诊断和病情监测作用。方法：收集70例系统性红斑狼疮（SLE）、114例类风湿性关节炎(RA)、55例强直性脊柱炎(AS)、及其年龄性别对应的健康血清标本。测定血清总ADA（tADA）、ADA1及ADA2活性。ROC曲线分析其诊断价值。结果：与健康对照相比,ADA活性在SLE患者血清中显著升高[tADA：15（12,20）vs 8（7,10）U/L;ADA1：3.5（2,5）vs 3（2,3）U/L;ADA2：11（8,15）vs 6（5,7）U/L;P<0.01）]。与健康对照相比,RA患者血清中tADA和ADA1活性无显著变化(P>0.05),ADA2活性水平升高[8（5.25,10）vs 7（5,9）U/L,P<0.05）];与健康对照相比,AS患者血清中tADA和ADA1活性无显著变化(P>0.05),ADA2活性升高[7（5,9）vs 6（5,7）U/L,P<0.05）]。ROC分析显示tADA及ADA2对SLE具有较好诊断价值（tADA：88.6%特异性、77.1%敏感性;ADA2：92.9%特异性、68.6%敏感性）。ADA活性对RA及AS患者无诊断价值。spearman相关性分析显示,tADA活性与SLE患者疾病活动度有一定正相关性（r=0.303,P=0.011)。结论：血清tADA活性检测可作为SLE辅助诊断和病情监测指标。     

Ubiquinone systems of the genus Cladosporium and morphologically similar taxa

Abstract We measured adenosine deaminase (ADA) activity in a guinea pig model of Legionella pneumophila infection. Female Hartley guinea pigs were inoculated intraperitoneally with one-quarter of the LD₅₀ dose of L. pneumophila Philadelphia-1 strain. Control groups were inoculated with clinical isolates of Staphylococcus aureus, Streptococcus pneumoniae, Haemophilus influenzae or Klebsiella pneumoniae . Each group consisted of 5 animals. ADA activity in plasma was assayed calorimetrically before and at various intervals after infection by measuring the amount of ammonia produced after adnosine was added to plasma samples. ADA activity before inoculation was 25.6±6.0 IU/1, it reached 174.4±60.0 IU/1 on day 3 after inoculation of L. pneumophila . ADA activity returned to normal levels on day 14. ADA activity did not increase significantly in guinea pigs infected with the other types of bacteria. These findings suggest that measurement of plasma ADA activity may be useful for the diagnosis of Legionella infection.     

Lack of adenosine deaminase activity in cultured murine cytotoxic T lymphocytes

The level of adenosine deaminase (ADA) activity was investigated in various populations of IL 2-dependent, cultured cytotoxic T lymphocytes (CTL), from bulk cultures as well as from CTL lines (CTL-A and CTL-B types). The study of C57BL/6 derived, cytotoxic bulk cultures yielded the following mean values of ADA activity: 12,500 U/mg in the cortical, immature region of the thymus, 1500 U/mg in the immunocompetent, cortisone-resistant medullary thymocytes, and 2000 U/mg in the T cell population from the spleen. These results are in agreement with previous studies on separated T lymphocyte populations of known origin and further indicate that a fall in ADA activity accompanies T cell maturation. ADA activity was measured in C57BL/6-derived CTL-A lines obtained from the thymic and splenic bulk cultures. All lines were characterized by a very low level of ADA activity, compared with the T cell bulk cultures freshly initiated from the thymic medulla or from the spleen, and to a variety of T tumor lines established in long term culture. Some showed undetectable ADA activity (less than or equal to 20 units/mg), whereas others maintained significant activity (50 to 500 U/mg). No correlation was found between the residual ADA activity level and the killing activity, at the time of the enzyme assay. Identical properties were observed for CTL-B cloned lines of various genetic backgrounds. These results suggest that the level of ADA activity of the CTL in the mouse is lower than the average value of mature T cells of the thymic medulla, and might constitute a differentiation marker specific to the CTL population. A possibility remains that low ADA activity levels in these CTL lines may be the consequence of an extinction of the ADA gene during in vitro growth, as it is observed for the cytotoxic activity itself. In either case, a low ADA activity level is a remarkable property of IL 2-dependent CTL clones, when compared to various established T tumor lines, which exhibit high and stable ADA levels during long term in vitro growth (5000 to 15,000 U/mg).     

Developmental expression of adenosine deaminase in placental tissues of the early postimplantation mouse embryo and uterine stroma

In this study, we have investigated the distribution of adenosine deaminase (ADA) in embryonic, extra-embryonic, and decidual tissues of the developing mouse embryo. ADA catalyzes a key step in purine metabolism converting adenosine to inosine. ADA specific activity (nmol/min/micrograms protein) was present at low levels in the embryo-decidual unit during the first 2 days of postimplantation development but then increased starting late on Day 6 of gestation (Day 0 plug). By Day 9, ADA specific activity was 80-fold higher than on Day 6. A histochemical staining method for ADA activity was applied to cryostat sections of the implantation site. The developmental increase localized primarily to the trophoblast/antimesometrial decidua interface between Days 7 and 9 of gestation, and decidua basalis and the metrial gland by Day 11. Immunofluorescent staining with sheep anti-mouse ADA antiserum confirmed the presence of ADA antigenicity in tissues forming the maternal/fetal interface. ADA specific activity was 19-fold higher in homogenates of the Day 11 decidua/parietal yolk sac than in the thymus, a tissue generally thought of as ADA-rich. High levels of ADA activity and immunoreactivity were also detected in the embryonal plasma during organogenesis, but the embryo proper showed only low levels. These results indicate that ADA is tightly regulated within tissues forming the maternal/fetal interface during early postimplantation stages of development.     

2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced reduction of adenosine deaminase activity in vivo and in vitro

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a potent immunosuppressant in several animal species. The purpose of this study was to determine if TCDD affected the activity of adenosine deaminase (ADA), a purine metabolizing enzyme that is vital to the proper functioning of the immune system. The effect of TCDD on ADA ctivity was studied in various tissues of male Balb/c mice (a TCDD-responsive strain) and DBA/2 mice (a less-responsive strain). Of the tissues examined after administration of TCDD in vivo (115 μ/kg, i.p.), ADA activity was found to be significantly reduced in thymic and splenic tissues of Balb/c mice at 24 hours postadministration. The enzyme activity in these affected tissues remained consistently low through 10 days postadministration. Such an effect of TCDD was both dose and time related in the thymic tissue of Balb/c mice. In contrast, no appreciable alterations in ADA activity were evident in any of the tissues of DBA/2 mice at any of the sampling intervals, indicating that such an effect of TCDD is likely to be mediated through the Ah receptor. This in vivo effect of TCDD on thymic ADA activity was also reproducible in situ where isolated whole thymuses were directly incubated with 10 nM TCDD. In this model, TCDD's effects on ADA activity were antagonized by known protein kinase or phosphorylation inhibitors such as quercetin, genistein, tyrphostin, and neomycin. These results indicate that the effect of TCDD on ADA activity in the thymus may be related to its property to elevate protein kinase activities in this tissue. ADA activity was also reduced in 3T3 cells that were treated with 10 nM TCDD in a low (1%) serum media. In contrast, 25 ng/mL epidermal growth factor (EGF) under such conditions consistently stimulated ADA activity. Interestingly, EGF at a similar concentration failed to elicit a stimulatory effect on ADA activity when cells were pretreated with TCDD. The property of TCDD to lower ADA activity under in vivo, in situ, as well as in vitro conditions appears to be largely related to its action to modulate protein phosphorylation activities.     

On the regulatory role of dipeptidyl peptidase IV (=CD=adenosine deaminase complexing protein) on adenosine deaminase activity

The molecular mechanism controlling the variable activity of the malignancy marker adenosine deaminase (ADA) is enigmatic. ADA activity was found to be modulated by the membrane-bound adenosine deaminase complexing protein (CP=DPPIV=CD26). The role of lipid-protein interactions in this modulation was sought. While direct solubilization of ADA in vesicles resulted in loss of ADA activity, the binding of ADA to CP reconstituted in vesicles restored the specific activity. The activity of ADA, free or bound to CP in solution, resulted in continuous linear Arrhenius plots. However, ADA bound to reconstituted CP exhibited two breaks associated with approximately 30% increased activity, at 25 and 13 degrees C, yielding three lines with similar apparent activation energies (E(a)). Continuum solvent model calculations of the free energy of transfer of the transmembrane helix of CP from the aqueous phase into membranes of various widths show that the most favorable orientations of the helix above and below the main phase transition may be different. We suggest that the 20% change in the thickness of the bilayer below and above the main phase transition may modify the orientation of CP in the membrane, thereby affecting substrate accessibility of ADA. This could account for ADA's reduced activity associated with increased membrane fluidity in transformed vs. normal fibroblasts.     

Adenosine-resistant chinese hamster fibroblast variants with hyperactive adenosine-deaminase: An analysis of the protection against exogenous adenosine afforded by increased activity of the deamination pathway

The activity of purine salvage and interconversion enzymes was examined in two sublines of Chinese hamster cells–RA11 and RA41–isolated on the basis of their resistance to adenosine concentrations toxic to wild-type CCL39 cells. Adenosine deaminase (ADA) activity was found to be two times higher in RA11 and three times higher in RA41 than in CCL39. Inhibition of ADA activity by coformycin reduced the level of adenosine resistance but did not restore wild-type sensitivity, indicating that a second defect contributes to the adenosine-resistant phenotype of these variants; evidence was indeed obtained for the presence in both lines of additional alterations protecting them against the lethal depletion of phosphoribosylpyrophosphate (Ishii and Green, 1973) imposed by adenosine to wild-type cells. To gain better insight into the influence of ADA hyperactivity on adenosine resistance, a procedure was developed for the specific isolation of variants with increased levels of ADA activity. Cell lines with 3–5 times and then 100–500 times the wild-type ADA activity were stepwise recovered. These investigations confirmed that amplification of ADA can efficiently contribute in protecting cells against high concentrations of exogenous adenosine. The variants isolated by this procedure again manifested, in addition to amplification of ADA activity, another alteration decreasing their sensitivity to adenosine. A possible mechanism accounting for the frequent isolation of variants that coexpress ADA hyper-activity and a second defect contributing protection against adenosine toxicity are considered.     

Human plasma adenosine deaminase 2 is secreted by activated monocytes

Adenosine deaminase (ADA) plays an important role in the immune system, and its activity is composed of two kinetically distinct isozymes, ADA1 and ADA2. ADA2 is a major component of human plasma total ADA activity and ADA2 activity is significantly elevated in patients with various diseases such as HIV infection and chronic hepatitis. However, relatively little is known about ADA2 because of difficulties in purifying this enzyme. In this study we succeeded in purifying human plasma ADA2 and demonstrate the extracellular secretion of ADA2 from human peripheral blood monocytes stimulated with phorbol 12-myristate 13-acetate and calcium ionophore.     

Adenosine deaminase activity in the human duodenal mucosa in relation to gastric acid secretion.

Adenosine deaminase (ADA) activity was estimated in mucosal specimens obtained endoscopically from the duodenal bulb. Three groups of subjects were studied: 1. 9 patients with achlorhydria, 2. 12 subjects with normal gastric acid secretion, 3. 5 patients with hypersecretion. Enzyme activity was measured by determination of ammonia liberated from the substrate according to the Chaney and Marbach method. In patients with hypersecretion the ADA activity was lower than in those with achlorhydria (p less than 0.001) and normal acid secretion (p less than 0.02). A significant negative correlation between ADA activity in the duodenal bulb mucosa and basal and maximal gastric acid outputs was found. The present study seems to indicate a possible relationship between gastric acid secretion and duodenal ADA activity.     

Reduced adenosine deaminase activity in the CNS of spontaneously-hypertensive rats

The activity of adenosine deaminase (ADA) has been measured in the hypothalamus, pons medulla and cerebral cortex from 30-day-old and 100-day-old spontaneously-hypertensive rats (SHR) and age-matched WKY controls. At 100 days there was a significant reduction in ADA activity in the hypothalamus (18.0%), pons medulla (20.6%) and cerebral cortex (14.7%). In 30-day-old SHR animals (prior to the development of significant hypertension) no significant changes were seen in the cerebral cortex or pons medulla but there was a small but significant reduction in ADA activity in the hypothalamus (9.2%). There was no significant reduction in the ADA activity in heart or kidney. Extracts of 100-day-old pons medulla which had been briefly heated to destroy endogenous ADA activity did not differentially affect the activity of exogenous purified ADA.     

The effect of benzene on the activity of adenosine deaminase in tissues of rats

Swiss Albino (Rat rattus norvegicus) rats were intraperitoneally injected with a 100 mg kg(-1) dosage of benzene, a toxic and carcinogenic agent widely used for industrial purposes. Changes in the adenosine deaminase (ADA) activity in the liver, kidney and serum of rats were investigated at 0, 2, 4, 8, 16, 32 and 64 h following injection. Serum physiological was administered to each control group. Enzyme activities were measured spectrophotometrically. Our purpose was to further investigations of some diseases caused by benzene, and present evidence of variations in the activity of ADA enzyme effected by benzene. While benzene caused significant inhibitions in ADA activity in the liver at 16 and 32 h and at 0.05 probability level, no significant inhibition or activation occurred at other test periods (hours). ADA activity did not present any significant variation in the kidneys. It was observed that ADA activity displayed similar patterns in the control groups. Comparisons of ADA activities in the two groups showed a statistically significant decrease between 4(th) and 64(th) hours (p< 0.05), demonstrating a direct correlation between benzene and its effects on ADA enzymes.     

2''-Deoxycoformycin Inhibition of Adenosine Deaminase in Rat Brain: In Vivo and In Vitro Analysis of Specificity, Potency, and Enzyme Recovery

2'-Deoxycoformycin (DCF), a potent inhibitor of adenosine deaminase (ADA), is increasingly used as a tool to investigate adenosine metabolism and neuromodulation. To advance further the usefulness of DCF for studies of purines in the CNS, we determined the inhibitory potency of this compound against ADA and adenylate deaminase (AMPDA) in brain, the rate of ADA recovery in various brain regions after single or repeated intraperitoneal DCF administrations, and the effect of DCF on several neurotransmitter synthetic enzymes. In vitro, the Ki values for inhibition of ADA and AMPDA were found to be 23 pM and 233 microM, respectively. In vivo, DCF inhibited ADA with ED50 values ranging from 155 to 280 micrograms/kg at 2 h posttreatment, and 98% inhibition was achieved with 1 mg/kg. AMPDA activity was not affected by doses up to 5.0 mg/kg. In contrast to the greater than 95% inhibition of ADA seen 1 day after DCF at 5 mg/kg, the effectiveness of a second similar DCF treatment on the activity that had recovered by 14 days was dramatically reduced. Eight days after DCF treatment with doses of 5-50 mg/kg, the degree of ADA activity recovery in 10 brain regions examined was similar; it averaged 35% of control values at the low dose but showed some heterogeneity, ranging from 15 to 54% of control values, at the higher doses. Forty days after treatment with a single dose of 5 mg/kg, ADA activity recovered by 68-78% of control values in brain regions with normally high levels of activity and by 44-59% of control values in other regions. The activities of choline acetyltransferase, glutamic acid decarboxylase, and histidine decarboxylase (an enzyme colocalized with ADA in hypothalamic neurons) were unaffected by DCF treatment, a result suggesting the lack of a generalized neurotoxic effect. The very low doses of DCF required for ADA inhibition in vivo are consistent with the high potency of this drug against ADA in vitro, and any physiological effects observed at low doses might therefore be ascribed to inhibition of ADA.