Human placental cytoplasmic 5'-nucleotidase: kinetics and molecular properties.
Explore the source record for details and available documents.
5'-Nucleotidase↗
Biomedical subjects
I H Fox
Publications and source records attributed to I H Fox.
Duchenne muscular dystrophy: normal ATP turnover in cultured cells.
We examined ATP metabolism in cultured muscle cells and fibroblasts from patients with Duchenne dystrophy. ATP and ADP levels were the same in cultured cells from normal subjects and patients and there was no difference in ATP synthesis or degradation. Although there was a significant decrease in radioactively labelled ATP after incubation with deoxyglucose in Duchenne muscle cells, there was no difference in ATP concentration or ADP metabolism.
Adenine Nucleotides↗
Effect of fructose infusion in hereditary xanthinuria.
Explore the source record for details and available documents.
Adult↗
Increased body fluid purine levels during hypotensive events. Evidence for ATP degradation.
Tissue ischemia leads to adenosine triphosphate (ATP) breakdown with elevation of body fluid ATP metabolites. This study tests the hypothesis that there is a direct relationship between periods of hypotension and body fluid uric acid and oxypurine levels in 19 prospectively studied patients. Significant elevations in urine oxypurine/creatinine clearance were found during periods of hypotension as compared with nonhypotensive periods (p less than 0.05). During severe episodes of hypotension, the serum urate level was significantly elevated as well (p less than 0.05). The increase in these body fluid products of ATP degradation may reflect cellular ischemia during hypotensive periods. There was a weak correlation (r = -0.31, p less than 0.001) between the systolic blood pressure and urine oxypurine/creatinine clearance. However, variability in the appearance of body fluid ATP breakdown products during episodes of hypotension suggests the interplay of multiple factors in the degradation of ATP. The use of ATP degradation products to quantitate the physiologic significance of clinical events remains tantalizing but not proved.
Adenosine Triphosphate↗
Inhibition of guinea-pig oxyntic cell function by adenosine and prostaglandins.
Adenosine and prostaglandins (PGs) are known inhibitors of oxyntic cell function. Using quantitative cytochemistry of hydroxyl ion production (HIP) in guinea-pig oxyntic cells, we examined the effects of adenosine and PGs on secretagogue-stimulated HIP. Adenosine (10(-6) M) inhibited the actions of histamine (10(-14) M) and gastrin (2.5 X 10(-12) M) by 69 and 67%, respectively, but not that of dibutyryl cyclic AMP (10(-16) M) or carbachol (10(-9) M). These observations suggest that adenosine does not influence the Ca++-dependent pathway of carbachol action and that the adenosine activity precedes the generation of cyclic AMP. Adenosine and related analogs, N6-L-phenylisopropyladenosine and 5-N-ethylcarboxam-idoadenosine (10(-12) to 10(-14) M), inhibited histamine-stimulated HIP (10(-14) M) in the following order: N6-L-phenylisopropyladenosine greater than 5-N-ethylcarboxamidoadenosine greater than adenosin. The adenosine antagonist, 1,3-diethylphenylxanthine (10(-6) M), reversed the inhibitory effects of adenosine. Exogenous PGE2 (10(-6) M) also inhibited histamine- and gastrin-stimulated HIP by 65 and 55%, respectively. Indomethacin (10(-6) M) and flurbiprofen (10(-6) M), PG synthesis inhibitors, potentiated the action of histamine by 175 and 159%, respectively. Adenosine was incapable of reversing this potentiated effect. These data indicate that adenosine and related analogs are inhibitors of oxyntic cell HIP and suggest that these biological properties are mediated by binding to a cell surface receptor and thereby regulating oxyntic cell adenylate cyclase activity. The more potent properties of N6-L-phenylisopropyladenosine as compared to 5-N-ethylcarboxamidoadenosine are consistent with activity at the high-affinity surface adenosine receptor.(ABSTRACT TRUNCATED AT 250 WORDS)
Adenosine↗
Regulation of deoxyadenosine and nucleoside analog phosphorylation by human placental adenosine kinase.
The enzymes responsible for the phosphorylation of deoxyadenosine and nucleoside analogs are important in the pathogenesis of adenosine deaminase deficiency and in the activation of specific anticancer and antiviral drugs. We examined the role of adenosine kinase in catalyzing these reactions using an enzyme purified 4000-fold (2.1 mumol/min/mg) from human placenta. The Km values of deoxyadenosine and ATP are 135 and 4 microM, respectively. Potassium and magnesium are absolute requirements for deoxyadenosine phosphorylation, and 150 mM potassium and 5 mM MgCl2 are critical for linear kinetics. With only 0.4 mM MgCl2 in excess of ATP levels, the Km for deoxyadenosine is increased 10-fold. ADP is a competitive inhibitor with a Ki of 13 microM with variable MgATP2-, while it is a mixed inhibitor with a Ki and Ki' of 600 and 92 microM, respectively, when deoxyadenosine is variable. AMP is a mixed inhibitor with Ki and Ki' of 177 and 15 microM, respectively, with variable deoxyadenosine; it is a non-competitive inhibitor with a Ki of 17 microM and Ki' of 27 microM with variable ATP. Adenosine kinase phosphorylates adenine arabinoside with an apparent Km of 1 mM using deoxyadenosine kinase assay conditions. The Km values for 6-methylmercaptopurine riboside and 5-iodotubercidin, substrates for adenosine kinase, are estimated to be 4.5 microM and 2.6 nM, respectively. Other nucleoside analogs are potent inhibitors of deoxyadenosine phosphorylation, but their status as substrates remains unknown. These data indicate that deoxyadenosine phosphorylation by adenosine kinase is primarily regulated by its Km and the concentrations of Mg2+, ADP, and AMP. The high Km values for phosphorylation of deoxyadenosine and adenine arabinoside suggest that adenosine kinase may be less likely to phosphorylate these nucleosides in vivo than other enzymes with lower Km values. Adenosine kinase appears to be important for adenosine analog phosphorylation where the Michaelis constant is in the low micromolar range.
Adenosine Diphosphate↗
Brain benzodiazepine receptor binding and purine concentration in Lesch-Nyhan syndrome.
Benzodiazepine receptor [( 3H]flunitrazepam) binding and purine concentration were measured in autopsied cerebral cortex of 4 patients who died with Lesch-Nyhan syndrome. Receptor density was normal in all 4 regions of Lesch-Nyhan cortex examined. However, an enhancement of benzodiazepine receptor affinity (25% reduction in Kd) was found in well-washed parietal and occipital cortex homogenates. Maximal gamma-aminobutyric acid (GABA) stimulation of [3H]flunitrazepam binding was normal in temporal, parietal and occipital cortex but markedly reduced (by 50-80%) in frontal cortex. Increased sensitivity to hypoxanthine inhibition (30% reduction in Ki) was also observed in parietal cortex. The concentrations of the purines hypoxanthine, xanthine and inosine in Lesch-Nyhan parietal cortex were about twice the values measured in control material matched for postmortem time. We suggest that the above-normal concentrations of purines estimated to be present in Lesch-Nyhan brain may be sufficient to significantly affect the ability of the benzodiazepine receptor to modulate GABA-mediated brain mechanisms.
Adolescent↗
Hyperuricemia and hypertriglyceridemia: metabolic basis for the association.
Hypertriglyceridemia has been reported frequently in patients with hyperuricemia and gout. The current studies have evaluated this relationship. To examine whether hypertriglyceridemia leads to hyperuricemia, IV intralipid was given to three gouty patients. Triglycerides increased from 169 to 700 mg/dl for three hours but caused no change in serum urate level or urine uric acid and oxypurine excretion. We next examined whether high carbohydrate intake increases serum urate and triglyceride levels. Four obese patients were placed on a 2000 kcal/d sucrose diet for seven days. The serum urate increased from 6.3 +/- 1.7 to 7.9 +/- 2.0 mg/dL. The percent uric acid clearance to creatinine clearance decreased from 5.9 +/- 1.3 to the lowest mean value of 3.7 +/- 1.2, while serum triglycerides increased from 106 +/- 33 to 252 +/- 57 mg/dL and blood lactate from 607 +/- 227 to 1167 +/- 381 mumol/L. A 3000 kcal/d glucose diet in four other obese subjects produced no change in serum urate levels but increased lactate and triglyceride levels. During an isocaloric sucrose diet in two normal men, the serum urate level increased from 5.3 and 4.0 to peak values of 9.5 and 7.4 mg/dL. The percent uric acid to creatinine clearance decreased from 5.6 and 6.6 to 2.9 and 3.3. The uric acid turnover did not increase. In three gouty patients the mean serum urate increased from 8.5 +/- 1.5 to 10.6 +/- 1.4 mg/dL following an isocaloric sucrose diet. The urine uric acid excretion increased from 0.30 and 0.25 to 0.37 and 0.38 mg/mg creatinine in two patients.(ABSTRACT TRUNCATED AT 250 WORDS)
Adult↗
Human placental nucleoside kinase activities.
Our studies have indicated that normal human placental cytosol contains a complex mixture of nucleoside kinase enzymes, some of which conform to previously characterized activities. Deoxyadenosine is phosphorylated by deoxycytidine kinase, adenosine kinase, and two as yet uncharacterized activities. Deoxyguanosine phosphorylation is associated with deoxycytidine kinase. More complete and detailed studies will be necessary to characterize these enzymes fully.
Adenosine Kinase↗
Hyperuricemia in glycogen storage disease type I. Contributions by hypoglycemia and hyperglucagonemia to increased urate production.
Studies were performed to determine whether hypoglycemia or the glucagon response to hypoglycemia increases uric acid production in glycogen storage disease type I (glucose-6-phosphatase deficiency). Three adults with this disease had hyperuricemia (serum urate, 11.3-12.4 mg/dl) and reduced renal clearance of urate (renal urate clearance, 1.1-3.1 ml/min). These abnormalities were improved in one patient by intravenous glucose infusion for 1 mo, suggesting a role for hypoglycemia and its attendant effects on urate metabolism and excretion. A pharmacologic dose of glucagon caused a rise in serum urate from 11.4 to 13.0 mg/dl, a ninefold increase in urinary excretion of oxypurines, a 65% increase in urinary radioactivity derived from radioactively labeled adenine nucleotides, and a 90% increase in urinary uric acid excretion. These changes indicate that intravenous glucagon increases ATP breakdown to its degradation products and thereby stimulates uric acid production. To observe whether physiologic changes in serum glucagon modulate ATP degradation, uric acid production was compared during saline and somatostatin infusions. Serum urate, urinary oxypurine, radioactivity, and uric acid excretion increased during saline infusion as patients became hypoglycemic. Infusion of somatostatin suppressed these increases despite hypoglycemia and decreased the elevated plasma glucagon levels from a mean of 81.3 to 52.2 pg/ml. These data suggest that hypoglycemia can stimulate uric acid synthesis in glucose-6-phosphatase deficiency. Glucagon contributes to this response by activating ATP degradation to uric acid.
Adenosine Triphosphate↗
Chronic allopurinol and adenine therapy in Duchenne muscular dystrophy: effects on muscle function, nucleotide degradation, and muscle ATP and ADP content.
Prompted by the controversy on the efficacy of allopurinol in Duchenne muscular dystrophy and by our observations of an abnormal adenine nucleotide turnover in this disease, we conducted an 18 month, double-blind clinical trial with allopurinol and adenine in 14 Duchenne boys paired according to age and functional activity. Detailed clinical evaluation was performed trimonthly. Muscle ATP and ADP content was measured before and after 1 year of treatment. The effect of therapy on adenine nucleotide turnover was determined. No significant difference was observed between the treated and placebo groups, but both showed a significant deterioration (p less than 0.05) in most clinical parameters. Muscle ATP was reduced in Duchenne dystrophy (p less than 0.02) but did not change with therapy, and no correction of the abnormal adenine nucleotide degradation was observed.
Adenine Nucleotides↗
Duchenne muscular dystrophy: normal ATP turnover in cultured cells.
We examined ATP metabolism in cultured muscle cells and fibroblasts from patients with Duchenne dystrophy. ATP and ADP levels were the same in cultured cells from normal subjects and patients, and there was no difference in ATP synthesis or degradation. Although there was a significant decrease in radioactively labeled ATP after incubation with deoxyglucose in Duchenne muscle cells, there was no difference in ATP concentration or ADP metabolism.
Adenosine Triphosphate↗
ATP degradation products after ischemic exercise: hereditary lack of phosphorylase or carnitine palmityltransferase.
We measured purine degradation products of ATP in plasma after ischemic exercise in eight normal subjects, one patient with myophosphorylase deficiency (McArdle's disease), and one with carnitine palmityltransferase deficiency. Normal subjects increase hypoxanthine and inosine, but not xanthine. Plasma purine levels were elevated above the normal range after ischemic exercise in McArdle's disease and in carnitine palmityltransferase deficiency after fasting. Those changes implied abnormally accelerated ATP degradation in these two myopathies.
Acyltransferases↗
Evidence for adenosine triphosphate degradation in critically-ill patients.
Alterations of cellular energy metabolism may provide important markers during the clinical course of critically ill patients. To determine whether adenosine triphosphate (ATP) degradation occurs in critically ill patients, we measured levels of adenosine, inosine, hypoxanthine, and xanthine in 18 patients and seven control subjects. The mean concentration of hypoxanthine (3.8 microM) in the critically ill patients was elevated (p less than 0.01) compared to that of our control population (0.1 microM). A subgroup of seven critically ill patients had levels of hypoxanthine, xanthine, or inosine higher than those of any member of the control group. This subgroup was characterized by a lower systolic blood pressure, an increased requirement for vasopressors, and a markedly decreased survival rate when compared to the other critically ill patients. Arterial and mixed venous blood gas values were not helpful in predicting survival and did not correlate with levels of ATP degradation products. In two patients who showed subsequent clinical improvement, the initially elevated levels of hypoxanthine and xanthine returned to normal. This study indicates that critically ill patients have elevated levels of ATP degradation products. These increased levels may indicate cellular hypoxia.
Adenosine↗
Characteristics of high-affinity and low-affinity adenosine binding sites in human cerebral cortex.
The binding characteristics of human brain cortical membrane fractions were evaluated to test the hypothesis that there are A1 and A2 adenosine binding sites. The ligands used were 2-chloro[8-3H]adenosine and N6-[adenine-2,8-3H]cyclohexyladenosine. Binding of chloroadenosine to human brain cortical membranes was time dependent, reversible, and concentration dependent. The dissociation constant (Kd) calculated for chloroadenosine by Scatchard analysis of equilibrium data was 280 nmol/L, with a maximum binding (Bmax) of 1.6 pmol/mg protein, suggesting a single class of binding sites. The specificity of chloroadenosine binding was assessed by the ability of adenosine analogues to compete for binding sites. With this approach, the apparent Kd was estimated to be 0.74 mumol/L for 5'-N-ethylcarboxamideadenosine, 1 mumol/L for cyclohexyladenosine, and 13 mumol/L for N6-(L-2-phenylisopropyl)adenosine. Isobutylmethylxanthine and theophylline, receptor antagonists, had apparent Kd values of 84 mumol/L and 105 mumol/L, respectively. Hill slope factors ranged from 0.3 to 0.6. Chloroadenosine binding to human brain cortical membranes approached equilibrium at 90 minutes, with a t 1/2 of 10 minutes. The kob was 0.080/min, and the k1 was 7.5 X 10(4)/min/mol/L. Reversibility of chloroadenosine binding at equilibrium was completed at approximately 10 minutes, with a k2 value of 0.074/min. The Kd calculated from the rate constants was 990 nmol/L. Cyclohexyladenosine binding was concentration dependent. The Kd calculated for cyclohexyladenosine via Scatchard analysis of equilibrium data was 5 nmol/L with a Bmax of 0.35 pmol/mg protein. Cyclohexyladenosine binding was displaced by three known receptor agonists: N6-(L-2-phenylisopropyl)adenosine (Kd 4 nmol/L), 2-chloroadenosine (Kd 10 nmol/L) and 5'-N-ethylcarboxamideadenosine (Kd 6 nmol/L).(ABSTRACT TRUNCATED AT 250 WORDS)
2-Chloroadenosine↗
Adenosine triphosphate degradation in specific disease.
Two hypotheses concerning adenosine triphosphate (ATP) degradation are examined: The metabolic basis of specific disorders involves ATP degradation, and uric acid and its precursors are measurable body fluid markers of ATP degradation. These hypotheses are examined by discussing human models of ATP degradation, methods for measurement of ATP degradation, and disorders of ATP degradation.
Adenine↗
Antihyperuricemic properties of amflutizole in gout.
The antihyperuricemic properties of amflutizole were investigated in studies designed to determine its efficacy and mechanisms of action in individuals with gout and hyperuricemia. In a randomized double blind, multiple dose, crossover study of 29 patients, amflutizole caused a significant dose dependent reduction in serum urate concentrations. Mean serum urate concentrations decreased significantly from 9.6 +/- 1.5 mg/dl to 7.2 +/- 1.3 mg/dl with the 500 mg dosage (p less than 0.01). Detailed studies in 5 patients demonstrated evidence for modest xanthine oxidase inhibition. However, the majority of the antihyperuricemic effect was derived from an enhanced renal clearance of uric acid. Although the drug has significant antihyperuricemic properties, these were inadequate to achieve adequate control of the serum urate concentration in hyperuricemia and gout at the doses utilized.
Adult↗
Enzyme replacement in the Lesch-Nyhan syndrome with long-term erythrocyte transfusions.
Explore the source record for details and available documents.
Blood Transfusion↗