Human serum xanthine oxidase: fluorometric assay applicable to the investigation of liver disorders

Quantification of uric acid, xanthine and hypoxanthine in human serum by HPLC for pharmacodynamic studies

A simple HPLC method was developed and validated for the determination of uric acid (UA), xanthine (X) and hypoxanthine (HX) concentrations in human serum to support pharmacodynamic (PD) studies of a novel xanthine oxidase inhibitor during its clinical development. Serum proteins were removed by ultrafiltration. The hydrophilic analytes and the I.S. were eluted by 100% aqueous phosphate buffer mobile phase. The hydrophobic matrix components (late peaks) were eluted with a step gradient of a higher organic mobile phase. Validation on linearity, sensitivity, precision, accuracy, stability, and robustness of the method for PD biomarkers (UA, X, and HX) was carried out in a similar manner to that for pharmacokinetic (PK) data where applicable. Issues of selectivity for endogenous biomarker analytes and individual concentration variations were addressed during method validation. Standards were prepared in analyte-free phosphate buffer. Quality control samples were prepared in control serum from individuals not dosed with the xanthine oxidase inhibitor. The method was simple and robust with good accuracy and precision for the measurement of serum UA, X, and HX concentrations.

Structure and function of xanthine oxidoreductase: where are we now?

Xanthine oxidoreductase (XOR) is a complex molybdoflavoenzyme, present in milk and many other tissues, which has been studied for over 100 years. While it is generally recognized as a key enzyme in purine catabolism, its structural complexity and specialized tissue distribution suggest other functions that have never been fully identified. The publication, just over 20 years ago, of a hypothesis implicating XOR in ischemia-reperfusion injury focused research attention on the enzyme and its ability to generate reactive oxygen species (ROS). Since that time a great deal more information has been obtained concerning the tissue distribution, structure, and enzymology of XOR, particularly the human enzyme. XOR is subject to both pre- and post-translational control by a range of mechanisms in response to hormones, cytokines, and oxygen tension. Of special interest has been the finding that XOR can catalyze the reduction of nitrates and nitrites to nitric oxide (NO), acting as a source of both NO and peroxynitrite. The concept of a widely distributed and highly regulated enzyme capable of generating both ROS and NO is intriguing in both physiological and pathological contexts. The details of these recent findings, their pathophysiological implications, and the requirements for future research are addressed in this review.

Serum xanthine oxidase in human liver disease

OBJECTIVES

High concentrations of serum xanthine oxidase (XO) have been reported during human liver disease and hepatocyte injury in experimental settings. However, it is unclear whether this elevation reflects hepatocyte necrosis or has a different meaning.

METHODS

The serum level of XO in 64 patients with chronic liver disease (17 patients with cirrhosis, 30 with chronic hepatitis, and 17 with cholestatic disorders) and in 12 control subjects was determined by a competitive ELISA. Conventional serum markers of liver damage were assessed in all patients, and grading and staging were scored in the chronic hepatitis group according to Knodell.

RESULTS

The XO serum levels were significantly higher in the patients than in the controls. The differences were also significant when controls were compared to patients with chronic hepatitis and cholestatic disorders separately, but not when compared to the cirrhosis group. Patients with cholestatic disorders had XO values higher than those of patients with cirrhosis or chronic hepatitis. XO levels did not correlate with stage and grade in chronic hepatitis group. We found a weak but significant positive correlation in patients between XO serum level and gamma-glutamyl transpeptidase (r = 0.37). This correlation was stronger when chronic hepatitis (r = 0.42) and, especially cholestatic disorders (r = 0.71), were separately tested, but was absent in the cirrhosis group. The XO values positively correlated with alkaline phosphatase in patients with cholestatic disorders. A level of serum XO >32 microg/ml specifically identified cholestatic disorders in our study population.

CONCLUSIONS

A marked elevation of serum XO in patients with chronic liver disease seems to reflect the presence of cholestasis. No correlation between XO levels and histological or serum evidence of hepatocyte necrosis was found in these patients.

Determination of xanthine oxidase in human serum by a competitive enzyme-linked immunosorbent assay (ELISA)

Xanthine oxidase was purified from human milk and used to immunise rabbits. A competitive immunoenzymatic assay with purified enzyme and rabbit antiserum was optimised to measure xanthine oxidase in human serum, the lowest detectable amount being 0.03 pmol of enzymatic protein. Thus, the test (i) is sensitive enough to determine xanthine oxidase in human serum, being more sensitive than the spectrophotometric method, (ii) it is more convenient for clinical laboratories than other sensitive tests and (iii) it has the advantage over the enzyme activity-based assays of also detecting inactive enzyme molecules. A competitive enzyme-linked immunosorbent assay (ELISA) was used to measure the serum xanthine oxidase level in healthy donors and in patients with liver diseases, and it was found that any concentration below 1 mg/L is in the normal range.

Xanthine oxidase, an indicator of secretory differentiation in mammary cells

Elevated levels of xanthine oxidase were found in (1) lactating mouse mammary glands, compared with virgin and midpregnant glands; and (2) primary mouse mammary cells cultured on floating collagen gels, compared with non-secretory cells on attached gels. In primary culture, increase in xanthine oxidase activity above a basal level coincided with secretory activity as measured by casein production; intracellular levels of casein and xanthine oxidase showed a high degree of correspondence. It is suggested that xanthine oxidase levels can be used as an indicator of in vivo and in vitro secretory differentiation in mammary epithelial cells.