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.

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.

Ethanol-induced activation of adenine nucleotide turnover. Evidence for a role of acetate

Consumption of alcohol causes hyperuricemia by decreasing urate excretion and increasing its production. Our previous studies indicate that ethanol administration increases uric acid production by increasing ATP degradation to uric acid precursors. To test the hypothesis that ethanol-induced increased urate production results from acetate metabolism and enhanced adenosine triphosphate turnover, we gave intravenous sodium acetate, sodium chloride and ethanol (0.1 mmol/kg per min for 1 h) to five normal subjects. Acetate plasma levels increased from 0.04 +/- 0.01 mM (mean +/- SE) to peak values of 0.35 +/- 0.07 mM and to 0.08 +/- 0.01 mM during acetate and ethanol infusions, respectively. Urinary oxypurines increased to 223 +/- 13% and 316 +/- 44% of the base-line values during acetate and ethanol infusions, respectively. Urinary radioactivity from the adenine nucleotide pool labeled with [8-14C] adenine increased to 171 +/- 27% and to 128 +/- 8% of the base-line values after acetate and ethanol infusions. These data indicate that both ethanol and acetate increase purine nucleotide degradation by enhancing the turnover of the adenine nucleotide pool. They support the hypothesis that acetate metabolism contributes to the increased production of urate associated with ethanol intake.

Altered cell cycle distributions of cultured human lymphoblasts during cytotoxicity related to adenosine deaminase inhibition

Serial-flow cytometric analysis of DNA content of T lymphoblasts (MOLT-4) and B lymphoblasts (MGL-8) was performed to correlate the cytotoxic properties of adenosine deaminase inhibition with alterations of DNA synthesis and disruptions of the cell cycle. The addition of deoxyadenosine up to 50 mumol/L potently decreased the growth of T lymphoblasts, and these changes were enhanced with the addition of 100 mumol/L homocysteine thiolactone. These conditions caused a virtual absence of cells from S and G2M phases after 24 hours. The DNA distribution was similar in cells cultured for 24 hours in 50 mumol/L deoxyguanosine or 2.5 mumol/L hydroxyurea. These observations suggested accumulation of cells in the G1 phase. T lymphoblasts cultured with up to 50 mumol/L adenosine had a substantial decrease in growth, which was not modified by the addition of homocysteine thiolactone. Cell cycle distributions of T lymphoblasts cultured for 24 to 48 hours under these conditions showed mild decreases in the G2M population. The addition of adenosine up to 50 mumol/L decreased the growth of B lymphoblasts, and these changes were enhanced by the addition of 100 mumol/L homocysteine thiolactone. These conditions induced mild decreases in the S-phase population in B lymphoblasts. The addition of deoxyadenosine, even with homocysteine thiolactone, did not modify growth in B lymphoblasts and the cell-cycle distributions were indistinguishable from distributions of control populations after 24 and 48 hours. The observations provide independent support for a reduction of DNA synthesis associated with cytotoxicity during adenosine-deaminase inhibition.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for an adenosine receptor in human tissues

Our observations suggest that [3H] chloroadenosine, an adenosine receptor agonist, identifies binding sites in human placenta with characteristics of the high affinity, adenosine receptor. The binding is time dependent, reversible and saturable. The potency series of adenosine receptor methylxanthine antagonists in displacing [3H] chloroadenosine is appropriate.

Ethanol induced alterations of uric acid metabolism

Our observations have shown that chronic oral ethanol administration was associated with increased serum urate, urine uric acid excretion, urine uric acid clearance and oxypurine excretion. The daily rate of uric acid turnover was significantly increased. Intravenous ethanol administration was associated with increased uric acid excretion, increased uric acid clearance and significantly increased oxypurine excretion. Excretion of radioactivity derived from intravenously administered adenine increased significantly. We conclude that hyperuricemia related to ethanol consumption at lower blood ethanol levels (less than 150 mg/dl) results from increased production of uric acid probably secondary to accelerated degradation of adenine nucleotides.

Human placental deoxyadenosine and deoxyguanosine phosphorylating activity

We studied deoxyadenosine and deoxyguanosine phosphorylating activities in human placental cytosol. The specific activities of nucleoside kinase enzymes in nanomoles per h per mg +/- SD were as follows: adenosine kinase, 30 +/- 14; deoxyadenosine kinase, 12 +/- 2; deoxycytidine kinase, 0.30 +/- 0.04; and deoxyguanosine kinase, 27 +/- 16. Three major activities were resolved by ion exchange and affinity chromatography: deoxyguanosine-deoxycytidine kinase, deoxycytidine-deoxyadenosine kinase, and adenosine-deoxyadenosine kinase. Two other activities contained significant quantities of deoxyadenosine kinase. Deoxyguanosine-phosphorylating activity eluted as a single peak in association with deoxycytidine kinase. This deoxyguanosine-deoxycytidine kinase had an apparent molecular weight of 54,000, a Stokes radius of 31 A, and apparent Km values of 10, 130, and 14 microM for deoxyguanosine, deoxycytidine, and ATP, respectively. Four peaks of deoxyadenosine phosphorylating activity were resolved by affinity chromatography with AMP-Sepharose 4B. Adenosine-deoxyadenosine kinase had an apparent molecular weight of 38,000, a Stokes radius of 27.4 A, and apparent Km values of 0.4, 510, and 75 microM for adenosine, deoxyadenosine, and ATP, respectively. Attempts to distinguish whether adenosine-deoxyadenosine kinase was one enzyme with these two activities or two separate enzymes suggested that the former was the case. Deoxycytidine-deoxyadenosine kinase had apparent Km values of 0.7, 670, and 12 microM for deoxycytidine, deoxyadenosine, and ATP, respectively. Its apparent molecular weight was estimated to be 49,000 and its Stokes radius 30 A. Two other minor peaks of deoxyadenosine-phosphorylating activity had characteristics different from either deoxycytidine kinase or adenosine kinase-associated deoxyadenosine kinase. Our studies indicate that human placental cytosol contains a complex mixture of nucleoside kinase enzymes.

Measurement of nucleoside kinases in crude tissue extracts

The measurements of deoxyadenosine kinase, adenosine kinase, and deoxycytidine kinase were examined in human placental cytosol to achieve a valid and reliable assay linear with time and protein. Our studies confirm the need to inhibit deaminase enzymes, since deoxyadenosine and deoxycytidine undergo extensive deamination and phosphorolysis. The use of a uniformly labeled nucleoside substrate introduced an artifact because the chromatographic behavior of the deoxyribose-1-phosphate, formed during the assay, was difficult to distinguish from the deoxynucleoside phosphate product. Accurate product identification was also essential. Finally, the substitution of GTP in place of ATP as the phosphate donor, the addition of a sulfhydryl reducing agent and a monovalent cation need to be considered when an assay is optimized. The use of these methods have lead to valid assays in placental cytosol that are linear with time and protein. Consideration of these important principles are necessary when establishing a valid and reliable nucleoside kinase assay in a crude tissue preparation.

Binding characteristics of an adenosine receptor in human placenta

The binding characteristics of human placental microsomes were evaluated for properties related to an adenosine receptor using 2-chloro[8-3H]adenosine as the ligand. Saturation of binding sites occurred with 0.75 pmol of ligand/mg of protein. Analysis of these data by Scatchard plot indicates a single class of binding sites with a Kd of 56 nM with 1.1 pmol of ligand bound/mg of protein. The specificity of 2-chloro[8-3H] adenosine binding was assessed by the ability of adenosine analogs to compete for binding sites. Using this approach, the Kd for 5'-N-ethylcarboxamideadenosine was estimated to be 0.3 microM and for N6-(L-2-phenylisopropyl)adenosine or N6-cyclohexyladenosine to be greater than 100 microM. Isobutylmethylxanthine and theophylline, receptor antagonists, have Kd values of 19 microM and 150 microM, respectively. Chloroadenosine binding to placental microsomes was time-dependent and reached equilibrium at approximately 20 min. The kob was 0.24 min-1 and the k1 was 0.71 X 10(8) min-1 M-1. Reversibility of chloroadenosine binding at equilibrium was completed at 5 min with a k2 value of 1.17 min-1. The Kd calculated from the reverse rate constant was 17 nM. Our observations suggest that 2-chloro[8-3H]adenosine identifies binding sites in human placenta with characteristics of an adenosine receptor. The binding properties conform to the previous description of a low affinity stimulatory receptor (Ra or A2 receptor).

Human hypoxanthine-guanine phosphoribosyltransferase

A mutant form of human hypoxanthine-guanine phosphoribosyltransferase (HPRTToronto) was isolated from erythrocytes of a male patient with gout due to a partial deficiency of enzyme activity. The tryptic peptides of HPRTToronto were mapped by reverse-phase high pressure liquid chromatography in an attempt to define the precise abnormality in its primary structure. Sequence analysis of the single aberrant peptide in HPRTToronto revealed an arginine to glycine amino acid substitution at position 50. A single nucleotide change in the codon for arginine 50 (CGA leads to GGA) could explain this substitution.

Ethanol-induced hyperuricemia: evidence for increased urate production by activation of adenine nucleotide turnover

Consumption of alcoholic beverages is associated with hyperuricemia and gout. To determine the contributions to this process of increased production and decreased excretion of uric acid, we gave oral ethanol (1.8 g per kilogram of body weight every 24 hours) for eight days or intravenous ethanol (0.25 to 0.35 g per kilogram per hour) for two hours to six patients with gout. During the long-term oral study we observed the following: serum urate levels increased from 8.4 +/- 0.4 (mean +/- S.E.) to 10.1 +/- 0.9 mg per deciliter; whole blood lactate reached a peak of 3.1 +/- 0.7 mM from a base line of 1.3 +/- 0.3 mM; and urinary oxypurines increased to 641 +/- 397 per cent of the base-line value. Urate clearance increased to 145 +/- 25 per cent of the base-line value. Daily uric acid turnover increased from 1010 mg per deciliter to 170 +/- 17 per cent of the base-line value. During short-term intravenous ethanol administration, serum urate levels, urate clearance, and urinary uric acid excretion were not substantially altered from the base-line period. Urinary oxypurine levels increased to 341 to 415 per cent of base-line values. Urinary radioactivity, originating from the adenine nucleotide pool labeled by [8-(14)C]adenine, increased to 127 to 149 per cent of base-line values. These data indicate that ethanol increases urate synthesis by enhancing the turnover of adenine nucleotides.

Regulation of purine metabolism by plasma membrane and cytoplasmic 5'-nucleotidases

The contribution of plasma membrane 5'-nucleotidase (E.C. 3.1.3.5) to intracellular purine degradation and release was evaluated in cultured human lymphoblasts. B-lymphoblasts and T-lymphoblasts are characterized by high and low levels of plasma membrane 5'-nucleotidase activity, respectively. After radiolabeling of the cellular adenine nucleotide pools with [8-14C]adenine, deoxyglucose-induced purine nucleotide degradation resulted in a 2-2.5 times greater release of cellular radioactivity from the B-lymphoblasts than from the T-lymphoblasts. Specific inhibition of plasma membrane 5'-nucleotidase with 50 microM alpha, beta-methylene adenosine diphosphate (AMPCP) did not decrease purine release during deoxyglucose-induced nucleotide degradation. Similarly, the inhibition of B-lymphoblast membrane 5-nucleotidase did not alter the incorporation of [8-14C]adenine into the nucleotide pool. Therefore, to explain the relatively high release of purine nucleotide degradation products in B-lymphoblasts when compared with T-lymphoblasts, cytoplasmic 5'-nucleotidase activity was investigated in these cell lines. B-lymphoblasts have seven times more cytoplasmic 5'-nucleotidase activity for dAMP and two to three times more activity for other purine nucleoside 5'-monophosphates than do T-lymphoblasts at pH 7.4. Membrane and cytoplasmic nucleotidase activities are produced by different enzymes that can be distinguished by differences in pH optima, Michaelis constants for purine substrates, divalent cation requirements, and susceptibilities to AMPCP inhibition. The data suggest that plasma membrane 5'-nucleotidase hydrolyzes extracellular nucleoside 5'-monophosphates only. Cytoplasmic 5'-nucleotidase most likely regulates the degradation of intracellular nucleoside 5'-monophosphates and may be responsible for the increased purine release observed in B-lymphoblasts.

S-Adenosylhomocysteine accumulation and selective cytotoxicity in cultured T- and B-lymphocytes

To evaluate for selective toxicity of S-adenosylhomocysteine toward cultured lymphoblasts, cytotoxicity was correlated with S-adenosyl-L-homocysteine accumulation in cultured human B-lymphoblasts (MGL-8) and T-lymphoblasts (MOLT-4) during adenosine deaminase inhibition with EHNA. The addition of adenosine increased intracellular S-adenosylhomocysteine levels and decreased the growth of B-lymphoblasts, with an estimated ID50 of 50 micro M. These changes were enhanced by the addition of homocysteine thiolactone. The addition of deoxyadenosine, even with homocysteine thiolactone, had no effect in B-lymphoblasts. The addition of deoxyadenosine potently decreased the growth of T-lymphoblasts, with an estimated ID50 of 16 micro M, and increased intracellular S-adenosylhomocysteine concentrations. The changes were enhanced with the addition of homocysteine thiolactone. T-lymphoblasts cultured with adenosine showed only modest increases in intracellular S-adenosylhomocysteine levels but did have a substantial decrease in growth. These changes were not substantially modified by the addition of homocysteine thiolactone. S-adenosyl-L-homocysteine hydrolase activity did not correlate with cytotoxicity or S-adenosyl-L-homocysteine accumulation in B- or T-lymphoblasts. These data suggest that selective S-adenosyl-L-homocysteine accumulation and toxicity in B-lymphoblasts provide a potential mechanism for the B-lymphocyte defect in adenosine deaminase deficiency. The accumulation of S-adenosylhomocysteine in T-lymphoblasts and the associated cytotoxicity provide evidence to implicate this mechanism as contributing to the T-cell disorders in inherited or acquired adenosine deaminase deficiency.

Hyperuricemia in acute illness: a poor prognostic sign

To clarify the role of the serum urate level and its change as a potential marker for severe tissue hypoxia, we have measured serum urate levels and urine uric acid excretion in 16 patients with acute cardiovascular disease. The six patients who died had a baseline mean serum urate level of 11.1 mg/lg (range, 6.6 to 15.5 mg/dl) and reached a peak mean value of 20.7 mg/dl (range, 13.6 to 33.0 mg/dl). Five of these patients had findings to suggest increased production of uric acid, in addition to decreased excretion of uric acid from impaired renal function. The 10 survivors had a baseline mean serum urate level of 6.8 mg/dl (range, 1.3 to 14.0 mg/dl) and a maximal mean peak value of 7.1 mg/dl (range, 2.9 to 14.0 mg/dl). There was no consistent evidence for increased production or decreased excretion of uric acid. Patients who died had a lower systolic blood pressure, arterial pH and plasma bicarbonate level and a higher heart rate and serum creatinine level compared with the patients ho survived. The observations suggest that marked hyperuricemia at the height of an illness may predict a fatal outcome. Tissue hypoxia may contribute to this sequence of events by leading to the depletion of adenosine triphosphate (ATP) and activation of purine nucleotide degradation to uric acid.

Inactivation of S-adenosylhomocysteine hydrolase during adenine arabinoside therapy

To assess for possible inhibition of cellular transmethylation during adenine arabinoside (ara-A) therapy, S-adenosylhomocysteine hydrolase activity was analyzed in 10 patients with chronic hepatitis B virus infection. In six patients receiving ara-A, enzyme activity was suppressed to 0-2% of control erythrocyte enzyme activity. This decrease in enzyme activity was evident within 4 h of starting the drug infusion and continued for 7 d after cessation of therapy. S-adenosylhomocysteine hydrolase activity of peripheral mononuclear cells was also measured in two patients receiving ara-A. Suppression to as low as 3.5% of pretreatment levels was found; however, marked fluctuations with partial return of enzyme activity during therapy was also observed in mononuclear cells. Inhibition of an enzyme involved in transmethylation reactions was observed in patients during ara-A therapy. This could contribute to the side effects and antiviral properties of ara-A.

Increased adenine nucleotide turnover in duchenne muscular dystrophy

To investigate a possible disorder of adenine nucleotide turnover in Duchenne muscular dystrophy, we evaluated 15 patients with mild Duchenne muscular dystrophy, eight patients with severe muscular dystrophy, seven patients with other neuromuscular disorders, and eight patients with hypogammaglobulinemia but no muscle disease. The serum urate concentration was similar in all four groups. Base line urinary purine excretion was elevated in all patients with neuromuscular disease with values of 1.72 +/- 0.15, 2.37 +/- 0.22, 2.49 +/- 0.35, and 2.60 +/- 0.48 mumoles/100 ml glomerular filtration for control subjects, mild Duchenne muscular dystrophy, severe disease, and other neuromuscular diseases, respectively. Adenine nucleotide pool turnover was measured by labeling with [8-14C]adenine and then 5 days later administering intravenous fructose. Five-day cumulative mean radioactivity excretion was elevated in mild and severe Duchenne muscular dystrophy with excretion values of 11.4 +/- 0.7 and 11.5 +/- 1.1% of administered radioactivity, respectively, as compared to 9.0 +/- 0.9% of administered radioactivity for control subjects. After intravenous fructose infusion, patients with Duchenne muscular dystrophy had a less than normal rise in serum urate concentration, a normal increase of urinary urinary purine excretion, and a greater than normal elevation of urinary radioactivity excretion and urinary purine specific activity. Patients with other neuromuscular diseases had virtually no rise in plasma urate concentration, less than normal increase in urinary total purine excretion, and a greater than normal increase of urinary radioactivity excretion and urinary specific activity. These observations suggest that there is an increased rate of adenine nucleotide turnover in Duchenne muscular dystrophy. In patients with other neuromuscular disease an increased rate of adenine nucleotide turnover resembled the abnormality expected from a diminished adenine nucleotide pool.

Biochemical evidence of dysfunction of brain neurotransmitters in the Lesch-Nyhan syndrome

Different brain regions were removed post mortem from three patients with the Lesch-Nyhan syndrome and were examined for alterations in hypoxanthine-guanine phosphoribosyl transferase (HGPRT), adenine phosphoribosyl transferase, and biochemical indexes of norepinephrine, dopamine, serotonin, gamma-aminobutyric acid (GABA), and acetylcholine neuron function, as compared with age-matched controls. The level of HGPRT activity in the material from patients with the Lesch-Nyhan syndrome was less than 1 per cent of control levels, whereas adenyl phosphoribosyl transferase was not significantly altered. All biochemical aspects of the function of dopamine-neuron terminals in the striatum (except dihydroxyphenylacetic acid levels) were decreased to 10 to 30 per cent of the control values. Serotonin and 5-hydroxyindoleacetic acid levels were increased, striatal choline acetyltransferase levels were low, and striatal glutamic acid decarboxylase and guanylate cyclase activities were unaltered. The disruption of the balance between the functions of GABA, dopamine, and acetylcholine neurons in the extrapyramidal system probably accounts for some of the symptoms observed in the Lesch-Nyhan syndrome (e.g., choreoathetosis).

Enhanced purine salvage during allopurinol therapy: an important pharmacologic property in humans

The contribution of enhanced purine salvage to the decreased total purine excretion associated with allopurinol therapy was measured by the intravenous administration of tracer doses of [8-(14)C] adenine to four patients with gout and normal purine salvage enzyme activity and four patients with the Lesch-Nyhan syndrome and absent purine salvage activity. The mean cumulative excretion of radioactivity 5 days after the adenine administration to patients not receiving and receiving (off and on) allopurinol therapy was 6.1% and 3.6% of infused radioactivity for gouty subjects and 15.9% and 20.8% for the Lesch-Nyhan patients. Urate pool size and urate turnover, as measured by pool labeling with [2-(14)C]uric acid, were substantially decreased in both groups of patients during allopurinol therapy. The intestinal loss of uric acid was estimated from these pool measurements on and off allopurinol. With a correction for this extrarenal purine loss, the mean cumulative excretions of radioactivity 5 days after adenine administration to patients off and on allopurinol therapy were 11.9% and 4.8% for the gouty subjects and 31.7% and 24.5% for the Lesch-Nyhan patients. In vitro studies demonstrated no alteration of the synthesis or degradation of adenine nucleotides by allopurinol in cultured human diploid fibroblasts. These observations suggest that enhanced purine salvage is an important component leading to decreased purine excretion during allopurinol therapy.