The effect of low-dose whole-body gamma-irradiation on the activity of enzyme systems in the rat adrenal medulla

A study was made of the pattern of changes in histochemical indices of key enzymes of the tricarboxylic acid cycle (succinate dehydrogenase) and glycolysis (lactate dehydrogenase) as well as of catecholamines (monoamine oxidase) in cells of the adrenal medulla of young and adult albino rats subjected to external whole-body gamma-irradiation with doses of 0.5 and 1.0 Gy (dose-rate of 2.7 X 10(-4) Gy/s). Radiosensitivity of the enzyme systems under study in the adrenal gland cells of young animals was higher than in that of adults. Changes of their levels in different periods of observation were mainly of phase nature and indicated the development of adaptation syndrome in the animal organism.     

Xanthine oxidase activities: evidence for two catalytically different types.

Xanthine dehydrogenase (EC 1.2.1.37) from mouse small intestine was accompanied by 20% as much xanthine oxidase (EC 1.2.3.2) activity (dehydrogenase-associated oxidase). NAD⁺ and oxygen did not compete as electron acceptors. Upon incubation at 37 °C, the dehydrogenase activity was gradually transformed to oxidase activity. Unexpectedly, the oxidase thus formed (dehydorgenase-derived oxidase) had catalytic properties different from those of the dehydrogenase-associated oxidase. The activation energy for the dehydrogenase-associated oxidase was 20,600 cal/mol, whereas that for the dehydrogenase-derived oxidase was 13,500 cal/mol. The activation energy for the dehydrogenase was 14,000 cal/mol. Between pH 6.4 and 8.5, the activity of the dehydrogenase-associated oxidase was essentially pH independent, whereas the activities of the dehydrogenase-derived oxidase and the dehydrogenase were enhanced with increasing pH. Use of the transformation inhibitor, dithiothreitol, and the protease inhibitor, diisopropylfluorophosphate, showed that these catalytic differences were not the result of partial proteolysis of the enzyme. The data demonstrate the existence of two catalytically different types of mammalian xanthine oxidase activities: A dehydrogenase-associated oxidase and a dehydrogenase-derived oxidase.     

Mechanisms of action of malondialdehyde and 4-hydroxynonenal on xanthine oxidoreductase

Studies have been made on the possible involvement of malondialdehyde (MDA) and (E)-4-hydroxynon-2-enal (HNE), two terminal compounds of lipid peroxidation, in modifying xanthine oxidoreductase activity through interaction with the oxidase (XO) and/or dehydrogenase (XDH) forms. The effect of the two aldehydes on XO (reversible, XO(rev), and irreversible, XO(irr)) and XDH was studied using xanthine oxidase from milk and xanthine oxidoreductase partially purified from rat liver. The incubation of milk xanthine oxidase with these aldehydes resulted in the inactivation of the enzyme following pseudo-first-order kinetics: enzyme activity was completely abolished by MDA (0.5-4 mM), while residual activity (5% of the starting value) associated with an XO(irr) form was always observed when the enzyme was incubated in the presence of HNE (0.5-4 mM). The addition of glutathione to the incubation mixtures prevented enzyme inactivation by HNE. The study on the xanthine oxidoreductase partially purified from rat liver showed that MDA decreases the total enzyme activity, acting only with the XO forms. On the contrary HNE leaves the same level of total activity but causes the conversion of XDH into an XO(irr) form.     

Effect of radiation on the xanthine oxidoreductase system in the liver of mice.

The xanthine oxidoreductase system is one of the major sources of free radicals in many pathophysiological conditions. Since ionizing radiations cause cell damage and death, the xanthine oxidoreductase system may contribute to the detrimental effects in irradiated systems. Therefore, modulation of the xanthine oxidoreductase system by radiation has been examined in the present study. Female Swiss albino mice (7-8 weeks old) were irradiated with gamma rays (1-9 Gy) at a dose rate of 0.023 Gy s(-1) and the specific activities of xanthine oxidase (XO) and xanthine dehydrogenase (XDH) were determined in the liver of the animals. The mode and magnitude of change in the specific activities of XO and XDH were found to depend on radiation dose. At doses above 3 Gy, the specific activity of XO increased rapidly and continued to increase with increasing dose. However, the specific activity of XDH was decreased. These findings are suggestive of an inverse relationship between the activity of XO and XDH. The ratio of the activity of XDH to that of XO decreased with radiation dose. However, the total activity (XDH + XO) remained constant at all doses. These results indicate that XDH may be converted into XO. An intermediate form, D/O, appears to be transient in the process of conversion. The enhanced specific activity of XO may cause oxidative stress that contributes to the radiation damage and its persistence in the postirradiation period. Radiation-induced peroxidative damage determined in terms of the formation of TBARS and the change in the specific activity of lactate dehydrogenase support this possibility.     

Xanthine oxidase activity in mouse pancreas: effects of caerulein-induced acute pancreatitis

The development of acute pancreatitis involves a number of pathophysiological changes which result in pancreatic tissue damage. Data from several models of acute pancreatitis suggest that the in vivo conversion of the enzyme xanthine dehydrogenase to xanthine oxidase may cause tissue damage by the subsequent generation of oxygen-derived free radical products. In the present studies, acute pancreatitis was induced in mice by the administration of supramaximal secretory doses of caerulein, a cholecystokinin analogue. Pancreatic xanthine oxidase activity was observed to occur in the dehydrogenase form in both control and treated mice. Artifactual conversion to the oxidase form could be induced by exclusion of 2-mercaptoethanol and phenylmethylsulfonyl fluoride from the buffer during tissue preparation. These data indicate that no significant conversion of xanthine dehydrogenase to oxidase is associated with this model of acute pancreatitis in mice.     

Proteolytic conversion of xanthine dehydrogenase to xanthine oxidase: evidence against a role for calcium-activated protease (calpain)

The present study tested the hypothesis that calpain is responsible for the limited proteolytic conversion of xanthine dehydrogenase (XD) to xanthine oxidase (XO). We compared the effects of various proteases on the activity and molecular weight of a purified preparation of xanthine dehydrogenase from rat liver. In agreement with previous reports, trypsin treatment produced a complete conversion of XD to XO accompanied by a limited proteolysis of XDH from an Mr of 140 kD to an Mr of 90 kD. Treatment with calpain I or calpain II did not produce a conversion from XD to XO nor did it result in partial proteolysis of the enzyme. Similarly, trypsin treatment partially degraded a reversibly oxidized form of xanthine dehydrogenase while calpain I or calpain II were ineffective. The possibility that an endogenous inhibitor prevented the proteolysis of XDH by calpain I or II was excluded by verifying that brain spectrin, a known calpain substrate, was degraded under the same incubation conditions. The results indicate that calpain is not likely to be responsible for the in vivo conversion of XD to XO under pathological conditions.     

Xanthine Oxidase is Not Responsible for Reoxygenation Injury in Isolated-Perfused Rat Heart

The massive leakage of intracellular enzymes which occurs during reoxygenation of heart tissue after hypoxic or ischemic episodes has been suggested to result from the formation of oxygen radicals. One purported source of such radicals is the xanthine oxidase-mediated metabolism of hypoxanthine and xanthine. Xanthine oxidase (O form) has been suggested to be formed in vivo by limited proteolysis of xanthine dehydrogenase (D form) during the hypoxic period (Granger el ai. Gastroenterology 81, 22 (1981)). We measured the activities of xanthine oxidase in both fresh and isolated-perfused (Langendorff) rat heart tissue. Approximately 32% of the total xanthine oxidase was in the O form in fresh and isolated-perfused rat heart. This value was unchanged following 60min of hypoxia and 30 minutes of reoxygenation. The infusion of 250/JM allopurinol throughout the perfusion completely inhibited xanthine oxidase activity but had no effect on the massive release of lactate dehydrogenase (LDH) into the coronary effluent upon reoxygenation of heart tissue subjected to 30 or 60min of hypoxia. Protection from 30min of hypoxia was also not obtained when rats were pretreated for 48 h with allopurinol at a dose of 30mg/kg/day and perfused with allopurinol containing medium. Superoxide dismutase (50 units/ml), catalase (200 units/ml), or the antioxidant cyanidanol (100μM) also had no effect on LDH release upon reoxygenation after 60 min of hypoxia. Xanthine oxidase activity was detected in a preparation enriched in cardiac endothelial cells while no allupurinol-inhibitable activity could be measured in purified isolated cardiomyocytes. It is concluded that xanthine dehydrogenase is not converted to xanthine oxidase in hypoxic tissue of the isolated perfused rat heart, and that the release of intracellular enzymes upon reoxygenation in this experimental model is mediated by factors other than reactive oxygen generated by xanthine oxidase.     

Xanthine Oxidase is Not Responsible for Reoxygenation Injury in Isolated-Perfused Rat Heart

The massive leakage of intracellular enzymes which occurs during reoxygenation of heart tissue after hypoxic or ischemic episodes has been suggested to result from the formation of oxygen radicals. One purported source of such radicals is the xanthine oxidase-mediated metabolism of hypoxanthine and xanthine. Xanthine oxidase (O form) has been suggested to be formed in vivo by limited proteolysis of xanthine dehydrogenase (D form) during the hypoxic period (Granger el ai. Gastroenterology 81, 22 (1981)). We measured the activities of xanthine oxidase in both fresh and isolated-perfused (Langendorff) rat heart tissue. Approximately 32% of the total xanthine oxidase was in the O form in fresh and isolated-perfused rat heart. This value was unchanged following 60min of hypoxia and 30 minutes of reoxygenation. The infusion of 250/JM allopurinol throughout the perfusion completely inhibited xanthine oxidase activity but had no effect on the massive release of lactate dehydrogenase (LDH) into the coronary effluent upon reoxygenation of heart tissue subjected to 30 or 60min of hypoxia. Protection from 30min of hypoxia was also not obtained when rats were pretreated for 48 h with allopurinol at a dose of 30mg/kg/day and perfused with allopurinol containing medium. Superoxide dismutase (50 units/ml), catalase (200 units/ml), or the antioxidant cyanidanol (100μM) also had no effect on LDH release upon reoxygenation after 60 min of hypoxia. Xanthine oxidase activity was detected in a preparation enriched in cardiac endothelial cells while no allupurinol-inhibitable activity could be measured in purified isolated cardiomyocytes. It is concluded that xanthine dehydrogenase is not converted to xanthine oxidase in hypoxic tissue of the isolated perfused rat heart, and that the release of intracellular enzymes upon reoxygenation in this experimental model is mediated by factors other than reactive oxygen generated by xanthine oxidase.     

Role of Lipoamide Dehydrogenase and Metallothionein on 1-Methyl-4-phenyl-1,2,3,6- tetrahydropyridine-induced Neurotoxicity

In the present study, we investigated the effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) on lipoamide dehydrogenase activity and metallothionein content. Lipoamide dehydrogenase is a flavoprotein enzyme, which reduces lipoamide and low molecular weight thiols. This enzyme has also been involved in the conversion of ubiquinone (coenzyme Q-10, oxidized form) to ubiquinol (reduced form). Lipoamide dehydrogenase activity was measured spectrophotometrically following its incubation with different doses of MPTP, MPP⁺, and divalent metals. MPTP at higher concentrations inhibited the lipoamide dehydrogenase activity, whereas it’s potent toxic metabolite 1-methyl-4-phenylpyridinium (MPP⁺) had a similar effect at lower concentration. Calcium and copper did not affect the enzyme activity at any of the doses tested, whereas, zinc dose dependently enhanced the lipoamide dehydrogenase activity. Additionally, levels of metallothionein in the mouse nigrostriatal system were measured by cadmium affinity method following administration of MPTP. Metallothionein content was significantly reduced in the substantia nigra (SN), and not in the nucleus caudatus putamen (NCP) following a single administration of MPTP (30 mg/kg, i.p.). Our results suggests that both lipoamide dehydrogenase activity and metallothionein levels may be critical for dopaminergic neuronal survival in Parkinson’s disease and provides further insights into the neurotoxic mechanisms involved in MPTP-induced neurotoxicity.     

Xanthine dehydrogenase from Drosophila melanogaster: purification and properties of the wild-type enzyme and of a variant lacking iron-sulfur centers.

Xanthine dehydrogenase has been purified to homogeneity by conventional procedures from the wild-type strain of the fruit fly Drosophila melanogaster, as well as from a rosy mutant strain (E89----K, ry5231) known to carry a point mutation in the iron-sulfur domain of the enzyme. The wild-type enzyme had all the specific properties that are peculiar to the molybdenum-containing hydroxylases. It had normal contents of molybdenum, the pterin molybdenum cofactor, FAD, and iron-sulfur centers. EPR studies showed its molybdenum center to be quite indistinguishable from that of milk xanthine oxidase. As isolated, only about 10% of the enzyme was present in the functional form, with most or all of the remainder as the inactive desulfo form. It is suggested that this may be present in vivo. Extensive proteolysis accompanied by the development of oxidase activity took place during isolation, but dehydrogenase activity was retained. EPR properties of the reduced iron-sulfur centers, Fe-SI and Fe-SII, in the enzyme are very similar to those of the corresponding centers in milk xanthine oxidase. The E89----K mutant enzyme variant was in all respects closely similar to the wild-type enzyme, with the exception that it lacked both of the iron-sulfur centers. This was established both by its having the absorption spectrum of a simple flavoprotein and by the complete absence of EPR signals characteristic of iron-sulfur centers in the reduced enzyme. Despite the lack of iron-sulfur centers, the mutant enzyme had xanthine:NAD+ oxidoreductase activity indistinguishable from that of the wild-type enzyme. Stopped-flow measurements indicated that, as for the wild-type enzyme, reduction of the mutant enzyme was rate-limiting in turnover. Thus, the iron-sulfur centers appear irrelevant to the normal turnover of the wild-type enzyme with these substrates. However, activity to certain oxidizing substrates, particularly phenazine methosulfate, is abolished in the mutant enzyme variant. This is one of the first examples of deletion by genetic means of iron-sulfur centers from an iron-sulfur protein. The relevance of our findings both to the roles of iron-sulfur centers in other systems and to the nature of the oxidizing substrate for the Drosophila enzyme in vivo are briefly discussed.     

Modulation of radiation-induced changes in the xanthine oxidoreductase system in the livers of mice by its inhibitors

The xanthine oxidoreductase (XOD) system, which consists of xanthine dehydrogenase (XDH) and xanthine oxidase (XO), is one of the major sources of free radicals in biological systems. The XOD system is present predominantly in the normal tissues as XDH. In damaged tissues, XDH is converted into XO, the form that generates free radicals. Therefore, the XO form of the XOD system is expected to be found mainly in radiolytically damaged tissue. In this case, XO may catalyze the generation of free radicals and potentiate the effect of radiation. Inhibition of the XOD system is likely to attenuate the detrimental effects of ionizing radiation. We have examined this possibility using allopurinol and folic acid, which are known inhibitors of the XOD system. Swiss albino mice (7-8 weeks old) were given single doses of allopurinol and folic acid (12.5-50 mg/kg) intraperitoneally and irradiated with different doses of gamma radiation at a dose rate of 0.023 Gy/s. The XO and XDH activities as well as peroxidative damage and lactate dehydrogenase (LDH) were determined in the liver. An enhancement of the activity of XO and a simultaneous decrease in the activity of XDH were observed at doses above 3 Gy. The decrease in the ratio XDH/XO and the unchanged total activity (XDH + XO) suggested the conversion of XDH into XO. The enhanced activity of XO may potentiate radiation damage. The increased levels of peroxidative damage and the specific activity of LDH in the livers of irradiated mice supported this possibility. Allopurinol and folic acid inhibited the activities of XDH and XO, decreased their ratio (XDH/XO), and lowered the levels of peroxidative damage and the specific activity of LDH. These results suggested that allopurinol and folic acid have the ability to inhibit the radiation-induced changes in the activities of XDH and XO and to attenuate the detrimental effect of this conversion, as is evident from the diminished levels of peroxidative damage and the decreased activity of LDH.     

Intra- and extracellular forms of ethanol-modified O-underglycosylated galactose oxidase

The effect of ethanol and tunicamycin on synthesis and secretion of galactose oxidase was studied in resting cells of Dactylium dendroides. Ethanol promoted an overall decrease in both intra- and extracellular enzyme levels to the same extent that it inhibited [14C]glucosamine incorporation into total protein. The carbohydrate content of the intracellular enzyme was also depressed (44%) with a simultaneous decrease in O-Ser linked oligosaccharides. The intracellular galactose oxidase obtained after exposure of mycelia to ethanol plus tunicamycin lost 86% of its carbohydrate moieties, whereas the extracellular form lost only 35%. In both cases, residual sugar moieties were not eliminated by mild alkaline treatment. These data suggest that ethanol affects O-glycosylation of galactose oxidase. O-Underglycosylation did not affect the S0.5 values for galactose but diminished the molar catalytic activity. The absence of O-Ser/Thr-linked saccharides turned the intracellular enzyme into a form more susceptible to proteolysis than that devoid of N-linked sugars (tunicamycin-treated). O-Underglycosylation had a significant effect on the renaturation-reactivation of the enzyme after denaturation with 2.4 M Gdn-HCl.     

Sulfhydryl oxidase-catalyzed conversion of xanthine dehydrogenase to xanthine oxidase

Xanthine oxidase may be isolated from various mammalian tissues as one of two interconvertible forms, viz., a dehydrogenase (NAD⁺ dependent, form D) or an oxidase (O₂ utilizing, form O). A crude preparation of rat liver xanthine dehydrogenase (form D) was treated with an immobilized preparation of crude bovine sulfhydryl oxidase. Comparison of the rates of conversion of xanthine dehydrogenase to the O form in the presence and absence of the immobilized enzyme indicated that sulfhydryl oxidase catalyzes such conversion. These results were substantiated in a more definitive study in which purified bovine milk xanthine oxidase, which had been converted to the D form by treatment with dithiothreitol, was incubated with purified bovine milk sulfhydryl oxidase. Comparison of measured rates of conversion (in the presence and absence of active sulfhydryl oxidase and in the presence of thermally denatured sulfhydryl oxidase) revealed that sulfhydryl oxidase enzymatically catalyzes the conversion of type D activity to type O activity in xanthine oxidase with the concomitant disappearance of its sulfhydryl groups. It is possible that the presence or absence of sulfhydryl oxidase in a given tissue may be an important factor in determining the form of xanthine-oxidizing activity found in that tissue.     

Quasi-elastic light scattering studies on pyruvate oxidase.

Quasi-elastic or dynamic light scattering has been used to examine the translational diffusion properties of the enzyme pyruvate oxidase (pyruvate: ferricytochrome beta 1 oxidoreductase, EC 1.2.2.2.). Controlled proteolysis of the enzyme converts the native form of the enzyme to a protease-activated form which has a specific activity about 20-fold greater than the native oxidase. Light scattering studies indicate no significant change in the size or shape of pyruvate oxidase as a result of this proteolytic activation. In both cases the enzyme may be characterized as a hydrated sphere with a Stokes radius of about 53A. The sedimentation velocity-diffusion technique was used to obtain the molecular weight of this tetrameric enzyme, about 252 000 with a value of f/f0 of 1.25.     

NADH dehydrogenase of Corynebacterium glutamicum. Purification of an NADH dehydrogenase II homolog able to oxidize NADPH

NADPH oxidase activity, in addition to NADH oxidase activity, has been shown to be present in the respiratory chain of Corynebacterium glutamicum. In this study, we tried to purify NADPH oxidase and NADH dehydrogenase activities from the membranes of C. glutamicum. Both the enzyme activities were simultaneously purified in the same fraction, and the purified enzyme was shown to be a single polypeptide of 55 kDa. The N-terminal sequence of the enzyme was consistent with the sequence deduced from the NADH dehydrogenase gene of C. glutamicum, which has been sequenced and shown to be a homolog of NADH dehydrogenase II. In addition to high NADH-ubiquinone-1 oxidoreductase activity at neutral pH, the purified enzyme showed relatively high NADPH oxidase and NADPH-ubiquinone-1 oxidoreductase activities at acidic pH. Thus, NADH dehydrogenase of C. glutamicum was shown to be rather unique in having a relatively high reactivity toward NADPH.     

The molybdoenzyme formylmethanofuran dehydrogenase from Methanosarcina barkeri contains a pterin cofactor

Recently formylmethanofuran dehydrogenase from the archaebacterium Methanosarcina barkeri has been shown to be a novel molybdo-iron-sulfur protein. We report here that the enzyme contains one mol of a bound pterin cofactor/mol molybdenum, similar but not identical to the molybdopterin of milk xanthine oxidase. The two pterins, after oxidation with I2 at pH 2.5, showed identical fluorescence spectra and, after oxidation with permanganate at pH 13, yielded pterin 6-carboxylic acid. They differed, however, in their apparent molecular mass: the pterin of formylmethanofuran dehydrogenase was 400 Da larger than that of milk xanthine oxidase, a property also exhibited by the pterin cofactor of eubacterial molybdoenzymes. A homogeneous formylmethanofuran dehydrogenase preparation was used for these investigations. The enzyme, with a molecular mass of 220 kDa, contained 0.5-0.8 mol molybdenum, 0.6-0.9 mol pterin, 28 +/- 2 mol non-heme iron and 28 +/- 2 mol acid-labile sulfur/mol based on a protein determination with bicinchoninic acid. The specific activity was 175 mumol.min-1.mg-1 (kcat = 640 s-1) assayed with methylviologen (app. Km = 0.02 mM) as artificial electron acceptor. The apparent Km for formylmethanofuran was 0.02 mM.     

The mechanism of conversion of rat liver xanthine dehydrogenase from an NAD+-dependent form (type D) to an O2-dependent form (type O).

Rat liver xanthine dehydrogenase, type D, has been isolated directly from crude extracts as an antibody complex and its properties compared with those of two oxidase forms of the enzyme, heat-treated type O and trypsin-treated type O, also isolated as antibody complexes. The type D antibody complex displays electron acceptor specificities and electron paramagnetic resonance properties characteristic of an NAD⁺-dependent dehydrogenase whereas the trypsin-treated type O complex behaves as an O₂-utilizing oxidase. The heat-treated type O complex displays intermediate behavior. After electrophoresis in dodecyl sulfate-urea-acrylamide gels, type D and heated type O enzymes show single polypeptide bands, each of approximately 150,000 molecular weight. The trypsinized type O also shows one major band but with an approximate molecular weight of 130,000. Purified type D enzyme, when proteolytically treated, is converted to an oxidase with increased mobility on polyacrylamide gels. The 150,000 molecular weight subunit is cleaved into smaller subunits during proteolysis. Treatment with 5,5′-dithiobis-(2-nitrobenzoic acid) converts the type D enzyme, whether isolated as the purified enzyme or as the immune precipitate, to type O enzyme in a time-dependent manner. Titration of type D and the two type O antibody complexes with 5,5′-dithiobis-(2-nitrobenzoic acid) reveals that type D and heated type O each has approximately 28 reactive sulfhydryls, whereas the trypsinized type O has only 8 such groups. Many of the free sulfhydryls are vicinal and form disulfide bonds during the conversion to an oxidase by this reagent. Unproteolyzed preparations of type O rat liver enzyme and milk xanthine oxidase are converted to type D enzymes by treatment with dithiothreitol. The converted enzymes display electron acceptor specificities and epr properties characteristic of an NAD⁺-dependent dehydrogenase molecule.     

Oscillations of enzyme activities during the cell-cycle of a glucose-repressed fission-yeast Schizosaccharomyces pombe 972h

1. Increased specific activities of cytochrome c oxidase, catalase, succinate dehydrogenase, succinate-cytochrome c oxidoreductase, NADH-cytochrome c oxidoreductase and malate dehydrogenase were observed during glucose de-repression of Schizosaccharomyces pombe. 2. The cell-cycle of this organism was analysed by three different methods: (a) harvesting of cells at intervals from a synchronous culture, (b) separation of cells by rate-zonal centrifugation into different size classes and (c) separation of cells by isopycnic-zonal centrifugation into different density classes. 3. Measurement of enzyme activities during the cell-cycle showed that all the enzymes assayed [cytochrome c oxidase, catalase, acid p-nitrophenylphosphatase, NADH-dehydrogenase, NADH-cytochrome c oxidoreductase, NADPH-cytochrome c oxidoreductase, succinate dehydrogenase, malate dehydrogenase, isocitrate dehydrogenase (NADP) and fumarate hydratase] show periodic expression as ;peaks'. 4. Cytochrome c oxidase shows a single maximum at 0.67 of a cycle, whereas succinate dehydrogenase exhibits two maxima separated by 0.5 of a cell-cycle. 5. All other enzymes assayed showed two distinct maxima per cell-cycle; for catalase, malate dehydrogenase and NADPH-cytochrome c oxidoreductase there is the possibility of multiple fluctuations. 6. The single maximum of cytochrome c oxidase appears at a similar time in the cycle to one maximum of each of the other enzymes studied, except for NADH dehydrogenase. 7. These results are discussed with reference to previous observations on the expression of enzyme activities during the cell-cycle of yeasts.     

Distribution of xanthine oxidoreductase activity in human tissues--a histochemical and biochemical study.

Localization of the activity of both the dehydrogenase and oxidase forms of xanthine oxidoreductase were studied in biopsy and postmortem specimens of various human tissues with a recently developed histochemical method using unfixed cryostat sections, poly-(vinyl alcohol) as tissue stabilizator, 1-methoxyphenazine methosulphate as intermediate electron acceptor and Tetranitro BT as final electron acceptor. High enzyme activity was found only in the liver and jejunum, whereas all the other organs studied showed no activity. In the liver, enzyme activity was found in sinusoidal cells and both in periportal and pericentral hepatocytes. In the jejunum, enterocytes and goblet cells, as well as the lamina propria beneath the basement membrane showed activity. The oxidase activity and total dehydrogenase and oxidase activity of xanthine oxidoreductase, as determined biochemically, were found in the liver and jejunum, but not in the kidney and spleen. This confirmed the histochemical results for these organs. Autolytic rat livers several hours after death were studied to exclude artefacts due to postmortem changes in the human material. These showed loss of activity both histochemically and biochemically. However, the percentage activity of xanthine oxidase did not change significantly in these livers compared with controls. The findings are discussed with respect to the possible function of the enzyme. Furthermore, the low conversion rate of xanthine dehydrogenase into xanthine oxidase during autolysis is discussed in relation to ischemia-reperfusion injury.