Genetic and environmental influences on adult attention deficit hyperactivity disorder symptoms: a large Swedish population-based study of twins.

BACKGROUND: Attention deficit hyperactivity disorder (ADHD) frequently persists into adulthood. Family and twin studies delineate a disorder with strong genetic influences among children and adolescents based on parent- and teacher-reported data but little is known about the genetic and environmental contribution to DSM-IV ADHD symptoms in adulthood. We therefore aimed to investigate the impact of genetic and environmental influences on the inattentive and hyperactive-impulsive symptoms of ADHD in adults. METHOD: Twin methods were applied to self-reported assessments of ADHD symptoms from a large population-based Swedish twin study that included data from 15 198 Swedish male and female twins aged 20 to 46 years. RESULTS: The broad heritability [i.e., A + D, where A is an additive genetic factor and D (dominance) a non-additive genetic factor] was 37% (A = 11%, D = 26%) for inattention and 38% (A = 18%, D = 20%) for hyperactivity-impulsivity. The results also indicate that 52% of the phenotypic correlation between inattention and hyperactivity-impulsivity (r = 0.43) was explained by genetic influences whereas the remaining part of the covariance was explained by non-shared environmental influences. These results were replicated across age strata. CONCLUSIONS: Our findings of moderate broad heritability estimates are consistent with previous literature on self-rated ADHD symptoms in older children, adolescents and adults and retrospective reports of self-rated childhood ADHD by adults but differ from studies of younger children with informant ratings. Future research needs to clarify whether our data indicate a true decrease in the heritability of ADHD in adults compared to children, or whether this relates to the use of self-ratings in contrast to informant data.

[Relapsed pulmonary lymphomatoid granulomatosis grade III: curative treatment with radioimmune therapy and autologous stem cell transplantation]. / Pulmonale lymphomatoide Granulomatose Grad III: kurativer Therapieansatz mit Radioimmuntherapie und autologer Stammzelltransplantation.

HISTORY AND ADMISSION FINDINGS: A 36-year-old woman presented with dyspnea, dry cough and severe chest pain. She had been treated for persistent shoulder pain for several months. Clinical findings were non-conclusive. Lactate dehydrogenase was slightly elevated, C-reactive protein was increased. INVESTIGATIONS: A chest X-ray and subsequent computed tomography (CT) scans showed a lesion measuring 10×8 cm in the right upper pulmonary lobe. A CT-guided biopsy was obtained and the histological examination revealed a dense lymphocytic infiltrate with abundant blasts in addition to areas of necrosis and fibrosis. These findings were consistent with lymphomatoid granulomatosis grade III. TREATMENT AND COURSE: After 6 cycles of rituximab, cyclophosphamide, doxorubicin, vincristin and prednisolone (R-CHOP), an early relapse developed which was treated with rituximab, ifosphamid, methotrexate and etoposide. Good partial remission was achieved after consolidating high-dose chemotherapy followed by autologous stem cell transplantation (ASCT). After two years of rituximab maintenance treatment positron emission tomography (PET-CT) revealed an increase of metabolic activity. A second high-dose therapy was then combined with Y-90 ibritumomab tiuxetan, which was well tolerated. During remission the previously present lymphoma lesion of the lung was resected. Histology did not reveal any residual active lymphomatoid granulomatosis. Complete remission has so far been maintained for one year after ASCT. CONCLUSIONS: Combination of Y-90 ibritumomab tiuxetan and high-dose chemotherapy followed by ASCT may offer an efficacious and well-tolerated targeted treatment approach for patients with relapsed lymphomatoid granulomatosis.

Reduced toxicological activity of cigarette smoke by the addition of ammonia magnesium phosphate to the paper of an electrically heated cigarette: subchronic inhalation toxicology.

Cigarette smoke is a complex chemical mixture that causes a variety of diseases, such as lung cancer. With the electrically heated cigarette smoking system (EHCSS), temperatures are applied to the tobacco below those found in conventional cigarettes, resulting in less combustion, reduced yields of some smoke constituents, and decreased activity in some standard toxicological tests. The first generation of electrically heated cigarettes (EHC) also resulted in increased formaldehyde yields; therefore, a second generation of EHC was developed with ammonium magnesium phosphate (AMP) in the cigarette paper in part to address this increase. The toxicological activity of mainstream smoke from these two generations of EHC and of a conventional reference cigarette was investigated in two studies in rats: a standard 90-day inhalation toxicity study and a 35-day inhalation study focusing on lung inflammation. Many of the typical smoke exposure-related changes were found to be less pronounced after exposure to smoke from the second-generation EHC with AMP than to smoke from the first-generation EHC or the conventional reference cigarette, when compared on a particulate matter or nicotine basis. Differences between the EHC without AMP and the conventional reference cigarette were not as prominent. Overall, AMP incorporated in the EHC cigarette paper reduced the inhalation toxicity of the EHCSS more than expected based on the observed reduction in aldehyde yields.

Expression of coronin-3 (coronin-1C) in diffuse gliomas is related to malignancy.

Coronin-3 (coronin-1C), a homotrimeric F-actin binding protein, has been shown to be important for cell migration and brain morphogenesis. Here, we present for the first time a detailed analysis of the expression pattern of coronin-3 in human brain tumours and demonstrate that coronin-3 expression correlates with malignant phenotype in diffuse gliomas. In general, the expression of coronin-3 varies in different brain tumour entities. However, in diffuse gliomas, the number of coronin-3 expressing tumour cells correlates with the degree of malignancy. High-grade gliomas, such as anaplastic astrocytomas, anaplastic oligodendrogliomas, anaplastic oligoastrocytomas and glioblastomas, show high numbers of tumour cells positive for coronin-3, while diffuse low-grade gliomas, such as diffuse astrocytomas, oligodendrogliomas and oligoastrocytomas, exhibit low numbers of coronin-3-positive tumour cells. In order to explore and verify a contribution of coronin-3 to the malignant phenotype of diffuse gliomas, we employed an efficient shRNA-mediated coronin-3 knockdown in U373 and A172 human glioblastoma cells. Coronin-3 knockdown glioblastoma cells exhibited reduced levels of cell proliferation, cell motility and invasion into extracellular matrix compared to control cells. Together, our findings demonstrate evidence for a contribution of coronin-3 expression in the malignant progression of diffuse gliomas.

13-HPODE and 13-HODE modulate cytokine-induced expression of endothelial cell adhesion molecules differently.

Expression of cellular adhesion molecules (CAMs) at endothelial surfaces represents a physiological response to vascular damage and mediates the initiation of inflammation and possibly of atherogenesis. The cytokines TNF alpha and IL-1 are potent inducers of CAMs in endothelial cells. Reactive oxygen species comprising lipid oxidation products have been implicated in the signaling pathways of both TNF alpha and IL-1 and accordingly could modulate atherogenic events. We, therefore, investigated the potential role of the lipoxygenase product, 13-hydroperoxyoctadecadienoic acid (13-HPODE), which has also been identified in oxidized low density lipoproteins on CAM expression in HUVEC. 13-HPODE induced the expression of ICAM-1 in a concentration dependent manner up to 75 microM. Higher concentrations were toxic. Similar effects were observed with H2O2 and phosphatidylcholine hydroperoxide. VCAM-1 and E-selectin were not induced by 13-HPODE. 13-HPODE administered simultaneously with IL-1 or TNF alpha induced ICAM-1 additively, suggesting that hydroperoxides and cytokines act on the same signaling pathways. In contrast, pretreatment of cells with 50 microM 13-HPODE for 1 hour rather inhibited subsequent cytokine-induced ICAM-1 and E-selectin expression. Surprisingly, the reduction product of 13-HPODE, 13-hydroxyoctadecadienoic acid (13-HODE) proved to be an even better inducer of ICAM-1 than 13-HPODE. Pretreatment with 13-HODE did not show any inhibitory effect on ICAM-1 expression. Our data show that lipoxygenase products differentially affect CAM expression. 13-HPODE is stimulatory by itself and can positively or negatively affect cytokine signaling depending on time of exposure. 13-HODE induces CAM expression by itself but does not inhibit cytokine signaling. Thus, the interplay of lipoxygenase products with proinflammatory cytokines can not simply be explained by an oxidant-mediated facilitation of cytokine signaling.

Inhibition of tumor necrosis factor-alpha- and interleukin-1-induced endothelial E-selectin expression by thiol-modifying agents.

The expression of endothelial-leukocyte adhesion molecules has been postulated to be regulated by redox-sensitive events. Tumor necrosis factor-alpha (TNF-alpha)- and interleukin-1 (IL-1)-induced E-selectin expression was analyzed after pretreating human umbilical vein endothelial cells with different thiol-modifying agents, ie, diamide, phenylarsine oxide, N-ethylmaleimide, and diethyl maleate. E-selectin protein expression was quantified by indirect immunofluorescence. All compounds suppressed the cytokine-induced E-selectin expression in a concentration-dependent manner, whereas the antioxidant N-acetylcysteine showed no effect. The inhibitory effect of diamide (100 micromol/L, 1 hour) was reversible within 6 hours when the cells were allowed to recover before application of cytokines. Reversibility was strongly delayed when cells were deprived of glutathione by buthionine sulfoximine pretreatment. Glutathione depletion alone did not influence cytokine-induced E-selectin expression. Analysis of cellular glutathione status showed a 3-fold increase in oxidized glutathione after diamide treatment. Monochlorobimane labeling also revealed a decrease in total cellular thiols. During recovery, the glutathione status was restored within 1 hour, whereas total thiol content and E-selectin expression needed at least 6 hours to return to baseline. Complete inhibition of E-selectin expression by the vicinal thiol blocker phenylarsine oxide (0.5 micromol/L) was reversed by dithiols like dithiothreitol or dimercaptopropanol, but not by the monothiol 2-mercaptoethanol. These data suggest that proteins with essential thiols, most probably vicinal thiols. are involved in the IL-1- and TNF-alpha-mediated induction of E-selectin. These thiols must be in the reduced state; oxidation or other modification thereof attenuates or abolishes the cells' response to the cytokines.

Determinants of PHGPx expression in a cultured endothelial cell line.

Selenoprotein biosynthesis may not only be affected by the availability of selenium and the transcription rate of pertinent genes but also by the activity of components of the selenocysteine incorporation complex, SelA, B, C, or D. Incorporation of selenocysteine into selenoproteins requires a complex co-translational mechanism guaranteeing the correct recoding of the termination codon TGA as selenocysteine codon. A particular tRNA(Ser)(Sec) is enzymatically transformed by selenophosphate into tRNA(Sec) which recognizes the UGA codon by means of a specific elongation factor (SelB) and a peculiar mRNA secondary structure. Selenophosphate is formed from selenide and ATP by the SelD gene product, selenophosphate synthase (SelD). To further elucidate the biological role of phospholipid hydroperoxide GPx (PHG-Px), we transformed cells with a heterologous (pig) PHGPx gene and/or an additional (human) SelD gene and studied the behaviour of these cells under selenium depletion and repletion. Transfection of the endothelial cell line ECV 304 with either PHGPx cDNA or SelD cDNA did not result in a substantial increase of PHGPx activities, independent of selenium supply. However, cells co-transfected with both, PHGPx and SelD cDNA, expressed significantly higher PHGPx activity. This effect was much more pronounced under selenium limiting conditions. The enhanced PHGPx activity correlated with two functional parameters, increased capability to reduce hydroperoxides and less sensitivity against H2O2-induced cytotoxicity. Thus, the ECV cells, stably transfected with PHGPx and SelD cDNA, provide a model to specifically investigate the role of PHGPx in endothelial cell function.

Interleukin-1-induced nuclear factor kappa B activation is inhibited by overexpression of phospholipid hydroperoxide glutathione peroxidase in a human endothelial cell line.

Oxygen radicals are commonly accepted mediators in the tumour necrosis factor-mediated nuclear factor kappa B (NF kappa B) signalling cascade, but evidence for their role during interleukin-1 (IL-1) signalling is lacking. To test the involvement of hydroperoxides we investigated whether IL-1-induced NF kappa B activation could be influenced by glutathione peroxidases (GPx). These enzymes remove hydroperoxides with various specificities for the hydroperoxide substrate. By overexpressing phospholipid hydroperoxide glutathione peroxidase (PHGPx), which characteristically reacts with lipophilic hydroperoxides, the roles of H2O2 and lipid hydroperoxides were assessed. A human umbilical endothelial cell line, ECV 304, was stably transfected with the genes for both PHGPx and selenophosphate synthetase (selD), which provides selenophosphate for selenoprotein biosynthesis. When grown in selenium-deficient culture medium, the double-transfected clone (ECVPHGPx+SelD+) expressed 5-fold higher (P<0.005) PHGPx activity (measured by phosphatidylcholine hydroperoxide removal) than controls. The rate of H2O2 removal was also significantly (P<0.01) higher in this clone. When grown with high levels of extracellular selenium (up to 100 nM selenite), PHGPx activity and H2O2 removal were enhanced substantially in control cells and transfected cells. Under these conditions, PHGPx activity was 1.7-fold (P<0.005) higher in ECVPHGPx+SelD+, but H2O2 removal was the same as in controls. IL-1-induced NF kappa B activation was inhibited by selenium supplementation in control cells. In ECVPHGPx+SelD+ under conditions of selenium restriction, IL-1 induced NF kappa B activation only to a similar extent as under conditions of selenium supplementation in controls, and activation was abolished with 50 nM sodium selenite. These results show that overexpressed PHGPx is sufficient to inhibit NF kappa B activation, and suggests that NF kappa B activation by IL-1 is mediated by a preferential substrate of PHGPx, such as a fatty acid hydroperoxide, rather than by H2O2, the preferred substrate of the more abundant cytosolic GPx.

Immunological characterization and activity of transglutaminases in human normal and malignant prostate and in prostate cancer cell lines.

Using biochemical assays, we compared enzyme activities with the immunoreactivity of antibodies against rat seminal transglutaminase (TGase), human erythrocyte TGase and guinea pig liver TGase in human normal prostate, primary prostatic carcinomas and prostatic carcinoma cell lines. Glandular cells of the epithelium were only exceptionally positive with the antibody against (rat) secretory TGase. Using the antibodies against tissue-type TGase, most immunoreactive cells were found in the basal cell layer of prostatic epithelium as well as in stroma (fibroblasts, endothelial cells), whereas immunoreactive glandular cells were sparse. In the case of benign prostatic hyperplasia, few, irregularly distributed secretory cells along with a small number of stromal cells were also immunoreactive with the tissue-type TGase antibody. In dedifferentiated carcinomas, immunoreactive cells were nearly completely absent. Of the prostate cancer cell lines, the LNCaP line showed neither TGase enzyme activity nor immunoreactivity, whereas the PC-3 cell line displayed significant enzyme activity and immunoreactivity. No hormone-dependent changes in either enzyme activity or immunoreactivity were recorded after in vitro treatment of the respective cell lines with estrogens, androgens and antiandrogens. As there is no correlation between androgen deprivation and TGase expression in nonmalignant and malignant human prostatic epithelial cells, TGase activity more likely indicates cellular lesions and consecutive repair mechanisms.

Tissue-type transglutaminase expression in the Dunning tumor.

Transglutaminases with different functions and tissue distribution patterns can be distinguished by specific antibodies and by inhibition of enzyme activity in the presence of guanosine triphosphate (GTP). The most common form is the so-called tissue-type transglutaminase that is apparently involved in membrane stabilization processes, e.g. during apoptosis, and can be inhibited by incubation with GTP at low calcium concentrations. A secretory transglutaminase that cannot be inhibited by GTP is synthesized in an androgen-dependent manner in the dorsal prostate of the rat, the site suggested to represent the origin of the Dunning tumor used as an experimental model in prostate cancer research. Here we studied the expression of transglutaminases in different Dunning tumor lines--mainly in the highly differentiated H subline--and characterized the enzyme both biochemically and immunocytochemically. A very high enzyme activity was found only in the less well differentiated HI-F tumor line. Immunohistochemical reactions and Western blot analysis showed that there is no secretory transglutaminase present in any of the Dunning tumor lines studied. Transglutaminase activity of the Dunning tumor results from the so-called tissue-type enzyme that is non-organ specific. The absence of a secretory form of transglutaminase does not support the contention of a prostatic origin of the Dunning tumor.

Functional properties of isolated stroma and epithelium from rat ventral prostate during androgen deprivation and estrogen treatment.

To identify the functional activities of prostatic stroma under different hormonal conditions, isolated stroma and epithelium from rat ventral prostate (RVP, intact or one week castrated or estrogen-treated), were studied in metabolic labeling experiments. Using a semiquantitative stereological procedure, the relative proportion of the epithelial and stromal compartment was determined in situ. In addition, the distribution of the androgen receptor was visualized by in situ hybridization and by immunocytochemistry. In castrated animals protein biosynthesis of the stroma and epithelium exceeded the control value by a factor 7 and 5, respectively. In estrogen-treated animals protein biosynthesis was reduced, reaching only between one tenth and one fifth of the control value. The amount of stroma obtained from these animals was very low. These results were confirmed by stereological findings and indicate a differential regulation of prostatic stroma and epithelium after estrogen challenge and androgen deprivation. Estrogen receptor was induced in epithelium and stroma in estrogenized animals whereas the androgen receptor was reduced in experimental specimens. During estrogenization the biosynthetic activity of both stroma and epithelium is depressed, while estrogen responsivity of the epithelium in terms of estrogen receptor expression is increased. Androgen withdrawal results in active transformation of the gland through increased stromal biosynthetic activity and epithelial regression.

[Axis-related spontaneous correction after tibial fractures in childhood]. / Achsenbezogenes Spontankorrekturverhalten nach Unterschenkelbrüchen im Kindesalter.

In the Department of Surgery at the Bergmannsheil University Hospital, a total of 50 children with measurable post-traumatic deformity of the axis after fracture of the lower limb were examined. Clinical and radiological monitoring was carried out 6 years after their accidents, which they had sustained at the age of 3-15 years. The real degree of axis deformity remaining was determined mathematically, and the correction tendency was analysed in three dimensions and projected graphically using sum vectors. This showed better results in the correction of axis deformations of the lower limb visualized on a-p X-ray photographs as varus or valgus than of deformations seen as ante- and recurvation of lateral X-ray photographs. The excentric arrangement of muscles in the dorsal part of the lower limb could be one reason for this. All 50 patients experienced a reliable spontaneous correction of the axis deformity with no complications of stimulatory growth disturbance with showing good functional results. Shaft fractures of the lower limb in children are generally treated conservatively, operative treatment being indicated only in the case of severe lesions of the soft tissue and in adolescent patients.

Two novel peripheral membrane proteins, pasin 1 and pasin 2, associated with Na+,K(+)-ATPase in various cells and tissues.

Purification of pig kidney Na+,K(+)-ATPase at low concentrations of SDS (0.5%) allowed copurification of several peripheral membrane proteins. Some of these associated proteins were identified as components of the membrane cytoskeleton. Here we describe two novel globular proteins of of Mr 77,000 (pasin 1) and Mr 73,000 (pasin 2) which copurify and coimmunoprecipitate with Na+,K(+)-ATPase and can be stripped off Na+,K(+)-ATPase microsomes by 1 M KCl. Pasin 1 and pasin 2 were detected by immunoblot analysis in various cells and tissues including erythrocytes and platelets. Immunostaining revealed colocalization of pasin 1 and Na+,K(+)-ATPase along the basolateral cell surface of epithelial cells of kidney tubules and parotid striated ducts (titers of pasin 2 antibodies were too weak for immunocytochemistry). In erythrocytes, pasin 1 and pasin 2 are minor components bound to the cytoplasmic surface of the plasma membrane. Pasin 1 showed the same electrophoretic mobility as protein 4.1b. However, both proteins have different isoelectric points (pasin 1, pI 6; protein 4.1, pI 7), different chymotryptic fragments, and are immunologically unrelated. Short pieces of sequence obtained from pasin 1 and pasin 2 were not found in any other known protein sequence. The occurrence of pasin 1 and pasin 2 in diverse cells and tissues and their association with Na+,K(+)-ATPase suggests a general role of these proteins in Na+,K(+)-ATPase function.

Demonstration of immunoreactive forms of erythrocyte protein 4.2 in nonerythroid cells and tissues.

Protein 4.2 is a major component of the erythrocyte membrane cytoskeleton. Here we show that immunoreactive forms of human (Mr 72,000) and pig (Mr 75,000) protein 4.2 are also associated with the plasma membrane of various nonerythroid cells and tissues, such as platelets, brain, and kidney. Protein 4.2 can be extracted from platelet membranes under the same conditions (pH 11, 1 M KI, 1 M urea) which are required to extract protein 4.2 from the erythrocyte plasma membrane. The demonstration of protein 4.2 in nucleated cells that contain also several other proteins of the erythrocyte membrane cytoskeleton indicates some general principles underlying the molecular construction of the plasma membrane in erythrocytes and nonerythroid cells.

Graft-versus-host reaction in small bowel transplantation and possibilities for its circumvention.

To describe GVHR in small bowel transplantation and its underlying mechanisms and to find methods for circumventing that response, accessory small bowel transplantation was carried out in the rat model. Animals not treated with cyclosporine, irradiation, or removal of the mesenteric lymph nodes of the graft died within 22 days postoperatively due to graft versus host disease. Mesenteric lymph nodes of the graft and recipient spleen and peripheral lymph nodes showed strong immunologic stimulation histologically and high antihost T-cell-mediated cytotoxic antihost reactivity. Seventy-one percent of the animals that had received 15 mg of cyclosporine per kilogram body weight orally survived 150 days after transplantation. After donor irradiation with 50 rads, 77 percent of the recipients survived 120 days. After microsurgical removal of the mesenteric lymph nodes of the graft, 89 percent survived 120 days. We conclude that GVHR plays an important role in small bowel transplantation and that the experimental regimens of donor, graft, and recipient treatment described herein have proved their efficacy for circumventing GVHR.

Turnover of mammalian phospholipids. Rates of turnover and metabolic heterogeneity in cultured human lymphocytes and in tissues of healthy, starved and vitamin A-deficient rats.

1. Choline- and inositol-labelled phospholipids of human cultured lymphocytes turn over in a biphasic manner; phytohaemagglutinin activation stimulates turnover. 2. Choline-labelled phospholipids of rat liver and kidney, but not of blood, turn over in vivo as fast as those of duodenum, ileum or colon. Turnover in the intestinal tissues is greater in fed than in starved or vitamin A-deficient rats. In each case phosphatidylcholine turns over relatively faster than sphingomyelin or lyso-phosphatidylcholine. 3. It is concluded that phospholipid turnover of the type described is a common feature of viable cells, and that metabolically favourable conditions increase, rather than decrease, turnover.

Analogues of ribose 5-phosphate and 5-phosphoribosyl pyrophosphate. The preparation and properties of ribose 5-phosphorothioate and 5-phosphoribosyl 1-methylenediphosphonate.

1. 5-Phosphoribosyl 1-methylenediphosphonate was isolated after reaction of ribose 5-phosphate and O-adenylyl methylenediphosphonate with 5-phosphoribosyl pyrophosphate synthetase from Ehrlich ascites-tumour cells. 2. The analogue reacted with adenine phosphoribosyltransferase, hypoxanthine phosphoribosyltransferase and nicotinamide phosphoribosyltransferase [K(m) (analogue)/K(m) (5-phosphoribosyl pyrophosphate) 0.17, 0.19 and 6.3 respectively; V(max.) (analogue)/V(max.) (5-phosphoribosyl pyrophosphate) 0.011, 0.26 and 1.1 respectively]. 3. The analogue was not a substrate for 5-phosphoribosyl pyrophosphate amidotransferase or orotate phosphoribosyltransferase. 4. Ribose 5-phosphorothioate was synthesized by allowing ribose to react with thiophosphoryl chloride in triethyl phosphate. The analogue was a substrate for 5-phosphoribosyl pyrophosphate synthetase from Ehrlich ascites-tumour cells. When this reaction was coupled to either adenine phosphoribosyltransferase or hypoxanthine phosphoribosyltransferase, adenosine 5'-phosphorothioate or inosine 5'-phosphorothioate was formed respectively.

Inhibition of 5'-nucleotidase from Ehrlich ascites-tumour cells by nucleoside triphosphates.

1. 5'-Nucleotidase activity was obtained in a soluble form after treatment of a particulate fraction from Ehrlich ascites-tumour cells with deoxycholate. The relative rates of hydrolysis of 6-thioinosine 5'-phosphate, UMP, AMP, CMP, GMP, IMP, xanthosine monophosphate, thymidine monophosphate and 2',3'-AMP were 180, 129, 100, 93, 83, 79, 46, 41 and 3 respectively. 2. Values found for the Michaelis constant were: AMP, 67+/-12mum; IMP, 111+/-8mum; GMP, 93mum. 3. ATP and thymidine triphosphate were competitive inhibitors of AMP hydrolysis (inhibitor constants 0.4 and 4.8mum respectively); UTP, GTP and CTP were mixed competitive and non-competitive inhibitors. Thymidine triphosphate was a competitive inhibitor of IMP hydrolysis (inhibitor constant 14.4mum) and ATP, UTP and GTP showed mixed competitive and non-competitive inhibition. 4. ATP, thymidine triphosphate, UTP, GTP and CTP did not completely inhibit hydrolysis of AMP, IMP and UMP; the concentrations of ATP required to inhibit AMP and IMP hydrolysis by 50% were 12 and 230mum respectively. 5. Non-hyperbolic curves relating activity to UMP concentration were obtained in the presence and absence of triphosphates. 6. After fractionation on Sephadex G-200 columns a single peak of 5'-nucleotidase activity (particle weight 120000-125000) was obtained with AMP, IMP and GMP as substrates. UMP hydrolysis was catalysed by enzyme in this peak and in two slower peaks corresponding to apparent particle weights of 32000 and 16000; a single component (particle weight 120000), reacting with UMP and insensitive to UTP inhibition, was obtained when the column was eluted with buffer containing 1mm-UMP. 7. The possible significance of the results in the regulation of tumour-cell 5'-nucleotidase is discussed.