The Mechanism of Dietary Alterations in Rat Hepatic Xanthine Oxidase Levels

Enzymes and the assessment of protein nutrition

This symposium is concerned with the assessment of protein nutritional status in man. The question we have to examine is whether measurements of enzyme activity are, or may be in the future, of any value for this purpose. There are several concepts which may be useful in considering this problem, such as the nature of enzymes and the regulation of their amount in the body, the effect on activity of a biochemical lesion at the cellular level, and the adaptive changes that reflect an altered balance of metabolic pathways.

The protective effect of caffeic acid phenethyl ester on ischemia-reperfusion injury in rat ovary

OBJECTIVE

This experimental study was designed to determine the changes in tissue levels of malondialdehyde, end-product of lipid peroxidation (MDA), reduced glutathione (GSH) and xanthine oxidase (XO) and the effect of caffeic acid (3,4-dihydroxycinnamic acid) phenethyl ester (CAPE) on these metabolite levels after adnexal torsion-detorsion model in rats.

METHOD

Forty adult female albino rats were divided into five groups: basal control (n = 8), sham operation (n = 8), torsion-detorsion plus saline (n = 8), torsion-detorsion plus CAPE (n = 8). and only torsion (n = 8). Rats in the sham operation group underwent a surgical procedure similar to the other groups but the adnexa was not torsioned. Rats in the torsion group were killed after 360 degrees clockwise adnexal torsion for 3 h and ovaries were harvested. CAPE was injected intraperitoneally 30 min before detorsion in the CAPE/detorsion group and saline was administered in the saline/detorsion group. After 3 h of adnexal detorsion, the rats in both groups were killed and adnexa were surgically removed.

RESULTS

MDA levels and XO activities in torsion-detorsion plus saline group increased significantly when compared to basal control, torsion and sham operation groups (P < 0.001). In the CAPE group, MDA levels and XO activities were lower than those of torsion-detorsion plus saline group, and differences between the two groups were statistically significant (P < 0.001). GSH levels in torsion-detorsion plus saline group were decreased significantly when compared to basal control and sham operation groups (P < 0.001). GSH levels in the CAPE group were higher than those of torsion-detorsion plus saline group, and differences between the two groups were statistically significant (P < 0.004). Morphologically, polymorphonuclear leukocytic infiltration and vascular dilatation were obvious in the ischemia-reperfusion damaged ovary, a change partially reversed by CAPE.

CONCLUSIONS

These results suggest that administration of CAPE has beneficial effects in the prevention of ischemia-reperfusion injury of the ovaries.

Renal cellular transport, metabolism, and cytotoxicity of S-(6-purinyl)glutathione, a prodrug of 6-mercaptopurine, and analogues

The disposition of S-(6-purinyl)glutathione (6-PG) and its metabolites, including the antitumor agent 6-mercaptopurine (6-MP), was characterized in freshly isolated renal cortical cells from male F344 rats to assess the ability of the kidney to convert 6-PG to 6-MP. The intracellular transport and accumulation of 6-PG and 6-MP, the metabolism of 6-PG to 6-MP, and the potential cytotoxicity of 6-MP, 6-thioxanthine (6-ThXan), and 6-thioguanine (6-ThGua) were determined. 6-PG and 6-MP were accumulated by renal cortical cells by time- and concentration-dependent processes, reaching maximal levels of 14.2 and 1.52 nmol/10(6) cells, respectively, with 1 mM concentrations of each compound. Treatment with acivicin, an inhibitor of 6-PG metabolism by gamma-glutamyltransferase, increased accumulation of 6-PG, and treatment with alpha-keto-gamma-methiolbutyrate, a keto acid cosubstrate that stimulates activity of the cysteine conjugate beta-lyase (beta-lyase), which generates 6-MP, decreased accumulation of 6-PG. Incubation of renal cells with 10 mM 6-PG generated 6-MP at a rate of 2.4 nmol/min per 10(6) cells, demonstrating that the beta-lyase pathway forms the desired product from the prodrug within the intact renal cell. Preincubation of cells with acivicin or aminooxyacetic acid, an inhibitor of the beta-lyase, decreased the net formation of 6-MP, demonstrating further the function of the beta-lyase. 6-MP, 6-ThXan, and 6-ThGua exhibited approximately equivalent cytotoxicity (45-55% release of lactate dehydrogenase with 1 mM at 2 hr) in isolated renal cells. Based on the known antitumor potency of these agents, this suggests that cytotoxicity and antitumor activity occur by distinct mechanisms. The high amount of accumulation of 6-PG and its subsequent metabolism to 6-MP, as compared with the relatively low amount of accumulation of 6-MP, in renal cells suggest that 6-PG can function as a prodrug and is a more effective delivery vehicle for 6-MP to renal cells than 6-MP itself. Administration of 6-PG may be an effective means of treating renal tumors or suppressing renal transplant rejection.

A re-evaluation of the tissue distribution and physiology of xanthine oxidoreductase

Xanthine oxidoreductase is an enzyme which has the unusual property that it can exist in a dehydrogenase form which uses NAD+ and an oxidase form which uses oxygen as electron acceptor. Both forms have a high affinity for hypoxanthine and xanthine as substrates. In addition, conversion of one form to the other may occur under different conditions. The exact function of the enzyme is still unknown but it seems to play a role in purine catabolism, detoxification of xenobiotics and antioxidant capacity by producing urate. The oxidase form produces reactive oxygen species and, therefore, the enzyme is thought to be involved in various pathological processes such as tissue injury due to ischaemia followed by reperfusion, but its role is still a matter of debate. The present review summarizes information that has become available about the enzyme. Interpretations of contradictory findings are presented in order to reduce confusion that still exists with respect to the role of this enzyme in physiology and pathology.

Lack of conversion of xanthine dehydrogenase to xanthine oxidase during warm renal ischemia

Irreversible transformation of xanthine dehydrogenase (XDH) to xanthine oxidase (XO) during ischemia was determined measuring XDH and total enzyme activity in kidneys before and after 60 min of clamp of the renal pedicle. Tissue levels of adenine nucleotides, xanthine and hypoxanthine were used as indicators of ischemia. After 60 min of clamping, ATP levels decreased by 72% with respect to controls whereas xanthine and hypoxanthine progressively reached tissue concentrations of 732 +/- 49 and 979 +/- 15 nmol.g tissue-1, respectively. Both total and XDH activities in ischemic kidneys (30 +/- 15 and 19 +/- 1 nmol.min-1.g tissue-1) were significantly lower than in controls when expressed on a tissue weight basis. The fraction of enzyme in the XDH form was however unchanged indicating that the reduction of the nucleotide pool is not accompanied by induction of the type-O activity of xanthine oxidase.

The metabolism of acetaldehyde and not acetaldehyde itself is responsible for in vivo ethanol-induced lipid peroxidation in rats

A single oral administration of ethanol (5 g/kg) to rats induced a marked increase in lipid peroxidation, in the liver and kidney within 9 hr, as assessed by malondialdehyde accumulation. The pretreatment with alcohol dehydrogenase (ADH) inhibitor, 4-methylpyrazole (1 mmol/kg) caused approximately 50% inhibition of the hepatic ADH activity and abolished this ethanol-induced lipid peroxidation. The disulfiram treatment (100 mg/kg) significantly inhibited 63% of the hepatic low Km aldehyde dehydrogenase (ALDH) but not the high Km ALDH. The cyanamide treatment (15 mg/kg) effectively decreased 83% of the low Km and 70% of the high Km ALDH in the liver. Although there was more than a 20-fold elevation of acetaldehyde levels by the inhibition of acetaldehyde metabolism with disulfiram or cyanamide, the ethanol-induced lipid peroxidation was significantly suppressed by pretreatment with these drugs. More than 90% inhibition of xanthine oxidase and dehydrogenase by the pretreatment with allopurinol (100 mg/kg), with no effect on the hepatic ADH and ALDH activities, did not alter the enhancement of lipid peroxidation following ethanol administration. We propose that the metabolism of acetaldehyde (probably via the low Km ALDH) and not acetaldehyde itself is responsible for the ethanol-induced lipid peroxidation in vivo and that the contribution of xanthine oxidase, as an initiator of lipid peroxidation through acetaldehyde oxidation is minute during acute intoxication.

Xanthine oxidase activity and immunologically detectable protein in the C57B1/6 mouse

1. Xanthine oxidase (XO) was purified from livers of C57B1/6 mice. Antibodies generated against the purified protein were used in an immunoassay to measure total XO protein. 2. Both the specific activity and amount of XO protein were greater in the proximal small intestine than in the liver. A pool of inactive enzyme was present in the small intestine which developed after weaning. 3. Male C57B1/6 mice had the same XO specific activity as females and neither the hepatic nor the intestinal XO activity were affected by the level of dietary protein. 4. When mice were fed diets with tungsten, XO activity was lost and the amount of XO protein in the small intestine was decreased.

Purification of xanthine dehydrogenase from rat liver: a rapid procedure with high enzyme yields

Xanthine dehydrogenase (EC 1.2.1.37) was purified approximately 1000-fold from liver homogenates of adult male Sprague-Dawley rats. Enzyme recovery was good (greater than 20% of the starting activity was obtained), and the homogeneously pure enzyme had a molecular mass of approximately 300,000 Da. The purified protein exhibited a specific activity of 2470 units/mg protein and spectral properties identical to those of the best preparations of this enzyme reported by other investigators. Routine preparations of this enzyme also possess higher dehydrogenase:oxidase ratios (typically between 5 and 6) than do other xanthine dehydrogenase preparations so far reported in the literature. Maximum dehydrogenase:oxidase ratios, greater than 10, could be obtained from this procedure if only peak dehydrogenase fractions from the chromatography columns were saved. The present small-scale purification method, which can be completed in 48-60 h, utilizes ammonium sulfate fractionation, Sephadex G-200 column chromatography, Blue Dextran-Sepharose column chromatography, and preparative gel electrophoresis.

Effects of allopurinol on myocardial ischemic injury induced by coronary artery ligation and reperfusion

The effects of allopurinol pretreatment (1 mg/ml in the drinking water for 7 days at an estimated daily dose of 75 mg/kg) on biochemical and chemical changes occurring following left circumflex coronary artery ligation (40 min) and reperfusion (60 min) were examined in pentobarbital-anesthetized rabbits. During the ischemic phase, allopurinol pretreatment provided significant preservation of cellular ATP levels and of mitochondrial ATP generation as compared with untreated animals (P less than 0.05). During the reperfusion phase, allopurinol pretreatment significantly prevented the decrease in left ventricular pressure, sodium and calcium accumulation and decreases in sarcolemmal Na+,K+-stimulated and sarcoplasmic reticulum K+,Ca2+-stimulated ATPase activities as compared with untreated animals (P less than 0.05). In contrast, the decrease in mitochondrial (azide-sensitive) ATPase during ischemia and the partial recovery during reperfusion were unaffected by allopurinol pretreatment. Our results indicate that the myocardial protective effects of allopurinol may differ mechanistically in the ischemic and reperfusion phases of injury. The fact that rabbit hearts do not contain detectable xanthine oxidase activity would seem to preclude an obligatory role of this enzyme both in the generation of myocardial ischemic/reperfusion injury and in the protective actions of allopurinol.

[3H]-nitrendipine binding in membranes obtained from hypoxic and reoxygenated heart

We compared the binding properties of [3H]-nitrendipine in heart membranes from normal guinea-pig heart and from hypoxic or hypoxic and reoxygenated heart. The [3H]-nitrendipine binds a single class of high capacity (Bmax 667.2 +/- 105.2) with high affinity (KD 0.14 +/- 0.02) binding sites. By contrast, in membranes of hypoxic and reoxygenated heart the Bmax decreases significantly while it remains unaffected during hypoxia. Xanthinoxidase activity is increased in hypoxic-reoxygenated hearts.

The presence of desulfo xanthine dehydrogenase in purified and crude enzyme preparations from rat liver

Crude and purified xanthine dehydrogenase preparations from rat liver were examined for the existence of a naturally occurring inactive form. Reduction of the purified enzyme by xanthine under anaerobic conditions proceeded in two phases. The enzyme was inactivated by cyanide, which caused the release of a sulfur atom from the molybdenum center as thiocyanate. The amount of thiocyanate released was almost in parallel with the initial specific activity. The active and inactive enzymes could be resolved by affinity chromatography on Sepharose 4B/folate gel. These results provided evidence that the purified enzyme preparation from rat liver contained an inactive form. A method for the determination of the active and inactive enzymes in crude enzyme preparations from rat liver was devised based on the fact that only active enzyme could react with [14C]allopurinol and both active and inactive enzymes could be immunoprecipitated quantitatively by excess specific antibody to xanthine dehydrogenase. The amount of [14C]alloxanthine (derived from [14C]allopurinol) bound to the active sulfo enzyme in crude rat liver extracts was about 0.5 mol/mol of FAD. As this content is closely similar to that in the purified enzyme, these results suggest the existence of an inactive desulfo form in vivo.

Quantitation of rat liver xanthine oxidase by radioimmunoassay. A mechanism for sex-specific differences

To further delineate the mechanism responsible for the differences in xanthine oxidase activity in male and female Sprague-Dawley rats, a sensitive and specific radioimmunoassay (RIA) was developed for the measurement of hepatic xanthine oxidase. The RIA could detect as little as 5 mg of liver enzyme. Specificity of the RIA was confirmed by 1) Ouchterlony double immuno-diffusion in which a single precipitin band exhibited xanthine oxidase activity, when crude liver homogenate and an enzyme-specific stain were used; 2) parallelism between purified 125I-labeled xanthine oxidase and serial dilutions of crude liver homogenate; 3) a linear correlation between xanthine oxidase activity and the level of enzyme protein; and 4) a single protein band coincident with purified xanthine oxidase, when an immunoprecipitate prepared from antisera and crude liver homogenate was analyzed on sodium dodecyl sulfate (SDS) polyacrylamide gels. Whether xanthine oxidase activity was assayed in the absence of nicotinamide adenine dinucleotide (NAD+) (oxidase form) or in the presence of NAD+ (dehydrogenase), male values were consistently higher, and both forms of the enzyme correlated significantly with each other. When purified to homogeneity, neither form of the enzyme was appreciably affected by 17 beta-estradiol or testosterone propionate. When the RIA was employed, levels of hepatic xanthine oxidase were significantly greater in male than in female rats. We concluded from these data that increased xanthine oxidase activity in the male corresponds to a greater quantitative complement of xanthine oxidase protein. Furthermore, lower xanthine oxidase activity in the female cannot be explained by immunologically cross-reactive material without enzyme activity nor by a direct sex-steroid enzyme interaction.

Rat hepatic xanthine oxidase activity: age and sex specific differences

The effect of maturation, castration, and sex hormonal treatment on hepatic xanthine oxidase activity (XOA) was evaluated in Sprague-Dawley rats. In mature rats XOA was 59% greater in males than in females, whereas in the immature animal the differences between sexes were insignificant. Pubescence resulted in a twofold increase in activity in the male which was prevented by prepubertal orchiectomy. Conversely, prepubertal oophorectomy caused a twofold increase in XOA in females. Urinary uric acid excretion paralleled XOA in rats fed 1% oxonic acid. XOA was significantly elevated in females after postpubertal castration or testosterone treatment while orchiectomy or estradiol-17 beta treatment in the mature male showed no effect. In summary, these data suggest that androgens are required during puberty for full expression of hepatic XOA, and furthermore, an ovarian suppressive effect is evident. In the adult rat only the female responds appreciably to hormonal manipulation.

Transport of bovine milk xanthine oxidase into mammary glands of the rat

This research investigated transport of bovine milk xanthine oxidase into mammary glands of the lactating rat. Transport capability suggested an exogenous, nonmammary, source for the enzyme. Five lactating rats were injected intracardially with 100 microgram of purified iodine-125 labeled xanthine oxidase and five were injected with 100 microgram of the enzyme unpurified. Four hours later the rodents were hand-milked, and radiation was confirmed in all samples by liquid scintillation counting. Counts were recorded per volume of milk and the percentage radiation was computed. Autoradiographs of the rats indicated radiation almost exclusively associated with the mammary glands. Greatest concentration of radioactivity was in the micellar casein fraction of milk, and a compound of high molecular weight, presumably [iodine-125]xanthine oxidase, was confirmed by gel filtration of the casein. Results suggest that the compound was transported into the mammary glands. The degree of transport was dependent upon the stage of lactation.

Studies on new intracellular proteases in various organs of rat. 1. Purification and comparison of their properties

1. Specific proteases which inactivate the apo-proteins of many pyridoxal enzymes were found in skeletal muscle, liver and small intestine of rats. The protease from these three organs were purified and their properties were compared. 2. The purified proteases from liver and skeletal muscle appeared homogeneous on acrylamide gel electrophoresis. Two different proteases were separated from small intestine. A homogeneous, crystalline enzyme was obtained from the muscle layer while enzyme from the mucosa was partially purified. 3. They showed substrate specificity for pyridoxal enzymes. Their pH optima were in an alkaline region. They showed activity with the substrate of chymotrypsin, N-acetyl-L-tyrosine ethyl ester, but not with that of trypsin, p-toluenesulfonyl-L-arginine ethyl ester. They were inhibited by pyridoxal phosphate or pyridoxamine phosphate and seryl residues were involved in their active center. 4. The four enzymes differed in the following characters: (a) molecular weights; (b) patterns of elution from a CM-Sephadex column; (c) rates of inactivation of substrate enzymes; (d) rates of cleavage of N-acetyl-L-tyrosine ethyl ester; (e) reactivities with antiserum against the enzyme from the muscle layer of small intestine; (f) specific activities. 5. The amino acid composition and effect of chemical modifications of the crystalline enzyme from the muscle layer of small intestine were examined to elucidate its active sites and mode of action. Serine and histidine residues were found to be essential for protease activity. A tyrosine residue was also necessary for activity. Modifications of its sulfhydryl group, amino residues and carboxyl group had no effect on its activity.

Purification and characterization of xanthine oxidase from livers of vitamin E deficient rabbits

Xanthine oxidase which increases in activity during vitamin E deficiency was purified from livers of deficient rabbits. The procedure incorporates preparative sucrose gradient centrifugation and yields a homogeneous preparation on acrylamide gel electrophoresis. The purified enzyme exhibits a pH optimum of 8.1 and a Km value of 22 muM. Gel filtration chromatography gave the molecular weight of 280 000. Acrylamide gel electrophoresis in the presence of sodium dodecylsulphate reveals two types of subunits of molecular weights 52 000 and 99 000.

Enzymes and the assessment of protein nutriton

This symposium is concerned with the assessment of protein nutritional status in man. The question we have to examine is whether measurements of enzyme activity are, or may be in the future, of any value for this purpose. There are several concepts which may be useful in considering this problem, such as the nature of enzymes and the regulation of their amount in the body, the effect on activity of a biochemical lesion at the cellular level, and the adaptive changes that reflect an altered balance of metabolic pathways.