Hypoxanthine urinary excretion

At the present time, we just report some experimental results of a study on the mechanism of action of allopurinol (U-hydroxy-pyrazolo (3, -d ) pyrimidine) and thiopurinol k thiopyrazolo (3, d) pyrimidine) on de novo biosynthesis of uric acid. In this present work, we have compared effect of alio and thiopurinol on oxypurine (xanthine and hypoxanthine) urinary excretion with their rate of synthesis of ribonucleotides in vitro by erythrocyte hemolysate in some particular enzymatic deficiencies (hypoxanthine-guanine phosphoribosyltransferase HGPRT, adenine phosphoribosyl-transferase APRT and xanthinuria). 

The nucleoside formed from hypoxanthine and ribose is known as inosine (Ino or I) and the corresponding nucleotide as inosinic acid. Further substitution at C-2 of -H by -OH and tautomerization yields xanthine (Xan). Its nucleoside is xanthosine (Xao, X). A similar hydroxylation at C-7 converts xanthine to uric acid, an important human urinary excretion product derived from nucleic acid bases. 

Purines are metabolized in a series of reactions involving hypoxanthine, xanthine, uric acid, and allantoin as end products that are subsequently excreted in urine. Fig. 3 shows the metabolic pathways for xanthine. Measurement of urinary excretion of purine metabolites, primarily allantoin or, additionally, uric acid, xanthine, and hypoxanthine, has been proposed as a marker for microbial... 

This autosomal recessive trait results in hypouricemia and in increased urinary excretion of hypoxanthine and xanthine. Patients frequently have xanthine stones. 

Using tungsten as a molybdenum antagonist, Higgins etal. (1956) demonstrated a 95-97% loss in rat intestine and liver xanthine oxidase activity, but without any notable adverse effect on the animals. However, in chicks - which use uric acid as the primary route of nitrogen excretion - oral administration of tungsten decreased urinary excretion of uric acid and caused a rise in fecal xanthine and hypoxanthine output, a reduction in growth, and an increase in mortality. 

Creatinine and purine derivatives—allantoin, uric acid, hypoxanthine, and xanthine present in biological samples— are important analytes for diagnoses of certain types of metabolic diseases and can serve as markers for these processes. Analyses for such substances are cmcial for diagnosis and the monitoring of renal diseases, metabolic disorders, and various types of tumorigenic activity. On the contrary, these compounds are very important in the field of animal nutrition, because the measurement of their urinary excretion is being used as an internal marker for microbial protein synthesis. 

Uric acid and xanthines are markers for metabolic disorders such as gout, Lesch—Nyman syndrome, and xanthinuria. Measurements of urinary excretion of purine metabolites, among them uric acid and xanthine, have been proposed as a marker for microbial protein synthesis. Their simultaneous determination is useful for diagnosis and treatment of hyperuricemia. In addition to xanthine and hypoxanthine, notable members of the xanthine class include caffeine, theophylhne, and theobromine. ... 

In vivo xanthine oxidase inhibition results in a reduction of uric acid (and allantoin) in urine and in an increase in the urinary excretion of hypoxanthine and xanthine as described by Elion 1 for allopurinol. The dose-dependence of the excreted amounts of hypoxanthine and xanthine is a relevant consideration when using these parameters to measure the action of uricostatics in the rat. The purpose of our studies was to obtain a simple and rapid method to detect the uricostatic quality of hypouricemic compounds. 

Fig.1 Urinary output of uric acid, hypoxanthine+xanthine, uracil and CAMP in 18 normal newborns on their first and fourth day of life. Mean and SD-range of the values. The 24-h. urinary excretions of the same substances measured in 4 newborns during and after E.T. are plotted. Numbers above the circles refer to the patient s number.

Manzke, H., Spreter v. Kreudenstein, P., Dorner, K., and Kruse, K., 1980, Quantitative measurements of the urinary excretion of creatinine, uric acid, hypoxanthine and xanthine, uracil, cyclic AMP and cyclic GMP in healthy newborn infants, Eur. J.Pediatr. 133 157... 

Fig. 1 shows the urinary excretion of allopurinol and its metabolites in subject 1(oxipurinol-7-riboside not determined). The administration of hypoxanthine as well as the other purines tested(table 2) caused a marked decrease of the excretion of allopurinol-1-riboside. This is most likely due to the increased formation of hypoxanthine (table 1) which can competitively inhibit the conversion of allopurinol by PNP or HGPRT, allopurinol-1-riboside... 

Fig. 1. Influence of dietary hypoxanthine(day 8-14) on the urinary excretion of allopurinol and its metabolites.

In patients with nearly complete deficiency of erythrocyte HGPRT activity (either Gout or LESH NYHAN syndrome) thiopurinol has no effect on plasma and urinary excretion of uric acid (DELBARRE et al. 1970), while in the same patients treated with allopurinol there is a rapid andimportant decrease of uric acid balanced by nearly stochiometric increase of oxypurines. Gouty patients with HGPRT deficiency have higher urinary oxypurine excretion with a more important contribution of hypoxanthine (H/X = 2,36) than... 

Xanthinuria is characterized by a large urinary excretion of oxypurine (xanthine + hypoxanthine) which replaces uric acid at the end product of purine metabolism. Patients with xanthinuria are very deficient in xanthine oxidase activity. This rare metabolic disorder may be of interest for both information ... 

We have given allopurinol and thiopurinol to a xanthinuric man and in this communication we report on the urinary excretion amount of U-6 dihydroxy PP oxipurinol, or i+-thio-6-hydroxy PP oxithiopurinol and the concomitant values of xanthine and hypoxanthine. 

The clinical case of this xanthinuric man has been reported by DELBARRE in a previous paper (to be published). This man is a pastry cook since he could not be in the hospital more than one or two days, most determinations were done without control diet. This explains the great difference of daily urinary oxypurine values that were found. This patient has normal renal function (creatinine clearance = 99 ml/min). His averaged uric acid and oxypurines (xanthine and hypoxanthine) plasma concentration were 1.2 and 0.58 mg per 100 ml. His 95 % range ( mean + 2s) urinary excretion of uric acid is 7 to 27 mg per day and of oxypurine 310 to 618 mg... 

We have found no modification of urinary excretion amount of oxypurines or of the ratio of hypoxanthine to xanthine after 3,7 and 57 days allopurinol (200 mg/days) and 3,5,1 and 25 days of thiopurinol(500 mg/days) administration, nor during the 2h hours following administration of a single dose of every PP. All the values found were in the 95 % range limits of the mean values obtained without treatment. 

Allopurinol, in contrast to the uricosuric drugs, reduces serum urate levels through a competitive inhibition of uric acid synthesis rather than by impairing renal urate reabsorption. This action is accomplished by inhibiting xanthine oxidase, the enzyme involved in the metabolism of hypoxanthine and xanthine to uric acid. After enzyme inhibition, the urinary and blood concentrations of uric acid are greatly reduced and there is a simultaneous increase in the excretion of the more soluble uric acid precursors, xanthine and hypoxanthine. 

In the absence of allopurinol, the dominant urinary purine is uric acid. During allopurinol treatment, the urinary purines include hypoxanthine, xanthine, and uric acid. Since each has its independent solubility, the concentration of uric acid in plasma is reduced and purine excretion increased, without exposing the urinary tract to an excessive load of uric acid. Despite their increased concentrations during allopurinol therapy, hypoxanthine and xanthine are efficiently excreted, and tissue deposition does not occur. There is a small risk of xanthine stones in patients with a very high urate load before allopurinol therapy this can be minimized by liberal fluid intake and urine alkalization. 

The initial plasma urate concentration (Table 1) was disproportionately high levels greater than 0.6-0.9mmol/l in cases of acute renal failure are rare. The plasma urate concentration remained high when renal function had returned to normal 115 ml/min) However, the urinary uric acid, hypoxanthine and xanthine were nomnal for age, as was the excretion of total oxypurines in relation to creatinine excretion on a millimolar basis. Only after clinical gout appeared two years later was purine overproduction demonstrated, with plasma and urinary urate increased, as were hypoxanthine and xanthine excretion. The oxypurine/creatinine ratio was raised. 

The supernatants were concentrated with CF 25 cone, washed twice with 1.0 ml buffer, and reacted with a reaction mixture containing 0.5 n mol/I hypoxanthine at 37°C for 30 minutes. Uric acid product was determined by the rate of increased absorption of 293 nm at 37°C measured in a Gilford 2400 type recording spectrophotometer. Protein concentration of the specimens was determined by the Lowry method. Urinary oxypurines were assayed by HPLC as follows. One to two ml of urine samples from 24 hours total excretion were diluted 5-10 times with 0.9% NaCl and filtrated by a 0.45 pm Millipore filter. Three quarters of ml of this sample with 0.25 ml of 0.1 N NaOH and 1.0 ml of ethyo-acetate n- Butanol =2 1 solution were mixed for one minute. 

Daily uric acid excretion was extremely low. Urinary hypoxanthine and xanthine were increased. Xanthine was 19.6 mg/dl, 11.3 mg/dl, 12.8 mg/dl and 26.1 mg/dl, respectively. 

Urinary uric acid, hypoxanthine, xanthine, allopurinol excretion assay by HPLC... 

Allopurinol has been used for 16 years at doses ranging from 200 to 800 mg/day for the control of primary and secondary hyperuricemia. At the most commonly used doses of allopurinol (300-400 mg/ day) about 70% of the allopurinol is oxidized to oxipurinol, which is excreted in the urine. Urinary allopurinol and allopurinol riboside each account for about 10% of the dose. Since the degree of xanthine oxidase inhibition is dose-related, not only the oxidation of hypoxanthine and xanthine to uric acid, but also the oxidation of allopurinol to oxipurinol might be expected to be strongly inhibited at high doses of allopurinol. This would lead to increased levels of allopurinol, as well as allopurinol riboside, in plasma and urine. The extent to which this phenomenon occurs was investigated in several laboratory animal species and in man. 

The urinary uric acid excretion and the urinary oxypurine excretion were measured up to 180 minutes after the start of the fructose infusion (Figure 4). A mean increase in the urinary uric acid excretion to 144% of control values and in urinary oxypurine excretion to 397% of control values occurred in the first hour after infusion. The marked rise in urinary oxypurines resulted primarily from a rise in hypoxanthine excretion. A striking increase in the excretion of inosine was also noted. No change was observed in the fractional clearance of uric acid. Pretreatment with allopurinol enhanced the absolute increase in urinary oxypurine excretion. These observations suggest that fructose-induced hyperuricemia is related to stimulation of uric... 

Allopurinol therapy in hyperuricaemic man has been shown to be advantageous from two points of view. Firstly, it reduces urinary uric acid excretion and increases the excretion of the precursor purines xanthine and, to a lesser extent, hypoxanthine. In addition, total urinary purine excretion (the sum of these three) may be reduced by as much as 0 during allopurinol therapy (1). This latter effect has been attributed to the formation of nucleotides of either hypoxanthine (1) or allopurinol itself (2), which in turn exert a feed back inhibitary effect on the first enzyme of de novo purine synthesis. 

We have shown that allopurinol and thiopurinol (about 200 mg per day) have no significant effet on the decrease of total urinary oxypurine excretion and on the ratio hypoxanthine to xanthine, either after prolonged period administration or after a single dose. It is well known that thiopurinol like allopurinol reduces the synthesis of uric acid in non xanthinuric subjects this effect needs normal activity of HGPRT. 

Xanthinuria is a rare hereditary disorder characterized by a gross deficiency of xanthine oxidase activity in tissues with a resultant decrease in urinary uric acid excretion and a concomitant increase in the excretion of xanthine and hypoxanthine in the urine. The differential diagnosis of hypouricemia includes many disorders such as uricosuric drugs, a specific defect in uric acid reabsorption from the tubule as reported by Praetorius and Kirk (1) and the Fanconi syndrome such as heavy metal intoxication or Wilson s disease. These are associated with an increase in uric acid excretion however while the association of hypouricemia and hypouricosuria in conjunction with xanthinuria is an expression of xanthine oxidase impairment either primary or induced by enzyme blockers such as allopurinol. 

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