A method for determining plasma levels of oxypurines

Thiopurinol: comparative enzyme inhibition and protein binding studies with allopurinol, oxipurinol and 6-mercaptopurine

1 The metabolites of thiopurinol have been investigated in intact human and pig erythrocytes using [6-(14)C]-thiopurinol, high voltage electrophoresis and automated cation exchange chromatography. 2 Pre-incubation of human erythrocytes in vitro with thiopurinol increased the formation of hypoxanthine from [8-(14)C]-inosine and simultaneously reduced IMP synthesis. 3 Reduced inosine formation, following one week of thiopurinol therapy, has also been observed in vitro using the intact red cells from three patients suffering from gout. 4 Binding of thiopurinol, and to lesser extents of 6-mercaptopurine, uric acid, oxipurinol and allopurinol, has been demonstrated electrophoretically to human and pig serum proteins. 5 Thiopurinol is shown to have a greater binding capacity for purified human serum albumin than uric acid or 6-mercaptopurine at varying substrate and albumin concentrations covering the physiological range. Under these conditions the binding of uric acid is reduced in the presence of thiopurinol. 6 A comparison of the distribution of thiopurinol, 6-mercaptopurine, allopurinol and oxipurinol in whole blood has revealed that allopurinol and oxipurinol are not irreversibly bound to cellular proteins; whereas 30% and 13% of the total cellular uptake was irreversibly bound in the case of thiopurinol and 6-mercaptopurine respectively at substrate levels of 1 nmol per μl blood.

Hypoxanthine production by ischemic heart demonstrated by high pressure liquid chromatography of blood purine nucleosides and oxypurines

An isocratic high pressure liquid chromatographic system was developed for the estimation of purine nucleosides and oxypurines in blood. Use was made of a reversed-phase column. Nucleotides derived from erythrocytes affected the separation; these compounds were removed with A12O3. The recovery of the whole clean-up procedure exceeded 75%, and the lower detection limit of the assay for blood metabolites was 0.1 mumol/l. In 6 healthy volunteers, non-resting, the following blood concentrations (mean values +/- S.D. in mumol/l) were observed: adenosine (less than 0.1), inosine (0.2 +/- 0.1), hypoxanthine (2.2 +/- 1.3) and xanthine (0.2 +/- 0.1). In plasma and serum the total amount of these compounds was 1.9 and 5.4 times higher, respectively, presumably due to nucleotide breakdown during blood processing. The myocardial arterial-venous differences of blood purine nucleosides, oxypurines and lactate were subsequently measured in blood samples from 13 patients with angiographically documented ischemic heart disease, undergoing an atrial pacing stress test. No significant release of adenosine, inosine and xanthine by the heart was detectable in this study. The myocardial arterial-venous difference of lactate changed from 0.01 +/- 0.03 mmol/l (mean +/- SEM) at rest, to -0.10 +/- 0.04 mmol/l during pacing (p less than 0.002). Relatively larger changes were observed for hypoxanthine: pacing increased the arterial-venous difference from -0.01 +/- 0.05 to -0.51 +/- 0.17 mumol/l (p less than 0.02). We conclude that the high pressure liquid chromatographic assay of blood hypoxanthine is a useful tool in the diagnosis of ischemic heart disease.

Reverse phase partition HPLC for determination of plasma purines and pyrimidines in subjects with gout and renal failure

A reverse phase partition mode of high pressure liquid chromatography was developed for the estimation of purine, pyrimidine and pyrazolopyrimidine nucleosides and bases. This method has been applied to the investigation of purine and pyrimidine metabolism in subjects with gout and/or renal failure during allopurinol therapy. Plasma levels of hypoxanthine and xanthine were quantitated in all subjects. No significant differences were observed between healthy males and gouty subjects with or without allopurinol medication. There was a significant increase in xanthine levels in patients with gout and renal failure, or renal failure on allopurinol, and plasma levels of oxipurinol were increased, but no correlation with glomerular filtration rate (GFR) was observed. The separation method was also used to examine urines from patients with known abnormalities of purine and pyrimidine metabolism in an attempt to evaluate its usefulness as a urine screening technique.

Growth limitation of BHK-21 cells and its relation to folate metabolism

The role of folate metabolism in growth control in monolayer and suspension cell cultures was studied in three related cell lines: BHK-21, polyoma-transformed BHK-21 (PyBHK), and an aminopterin-resistant derivative of BHK-21 (A5). BHK-21 cells had extremely low levels of dihydrofolate reductase, PyBHK had higher levels, and A5 had extremely high levels. Hypoxanthine and thymidine together, but not individually, induced BHK-21 to grow in agar, and stimulated its growth in agarose and monolayer culture. PyBHK and A5 grew spontaneously in agar, and hypoxanthine plus thymidine had little or no effect on their growth either in suspension or in monolayer cultures. We found that exogenous folinic acid, a derivative of folate metabolism that bypasses the function of dihydrofolate reductase, mimicked the growth-stimulatory effects of exogenous hypoxanthine plus thymidine BHK-21. We conclude that the growth limitation of BHK-21 in suspension culture is due, in part, to a deficiency of dihydrofolate reductase. This enzyme deficiency limits nucleoside synthesis and can be overcome by supplying end products of this pathway.

Changes in coronary venous inosine concentration and myocardial wall thickening during regional ischemia in the pig

Correlations between local mechanical and metabolic events were studied during a partial decrease in flow in the left anterior descending coronary artery in 14 open-chest pigs. A decrease in flow to 28% (19-39%) of the control value was achieved with an adjustable screw clamp. A flow probe was placed around the artery. Blood samples were taken from the regional anterior coronary vein and the femoral artery. Myocardial wall thickening was measured with a harpoon type of mercury strain gauge. During ischemia, systolic myocardial wall thickening decreased to 44.5 ± 5.1% ( SE ) of its control value ( P < 0.001). The mean concentrations of plasma potassium and whole blood inosine, hypoxanthine, and lactate in three serial 2-minute samples obtained during a 6-minute control period were compared with those obtained during a 6-minute period of partial occlusion. During ischemia, venous inosine concentration increased from 10.9 ± 0.7 µ M to 18.5 ± 1.8 µM ( P < 0.005), venous hypoxanthine concentration increased from 28.5 ± 1.4 µ M to 33.0 ± 1.5 µ M ( P < 0.005), venous potassium concentration increased from 3.77 ± 0.10 m M to 4.08 ± 0.13 m M ( P < 0.001), and venous lactate concentration increased from 1.04 ± 0.19 m M to 1.52 ± 0.17 m M ( P < 0.001). The arterial level of potassium increased very little. The arterial concentration of the other compounds did not change significantly during the 6-minute period of ischemia. Myocardial lactate extraction changed from a control value of 42.6 ± 6.7% to -4.6 = 12.5% ( P < 0.05). A negative correlation ( r s = -0.79, P < 0.01) was observed between venous inosine concentration and myocardial wall thickening (percent of control) during ischemia. This study indicates that the local venous inosine concentration is a sensitive indicator of regional myocardial ischemia in the pig.