Oxidation of synephrine by type A and type B monoamine oxidase.

Synephrine (SP) was found to be a substrate for monoamine oxidase (MAO) in rat brain mitochondria, showing the Km and Vmax values of 250 microM and 32.6 nmoles/mg of protein/30 min respectively. The inhibition studies showed that the SP oxidation was carried out by both type A and type B MAO and a major part of the activity was due to type A MAO.     

Monoamine oxidase inhibition by d-amphetamine in ganglia and nerve endings

Summary The inhibition of monoamine oxidase (MAO) activity by d-amphetamine was measured in homogenates of cat superior cervical ganglion and nictitating membrane, using tyramine (TM) and noradrenaline (NA) as substrates. In both tissues, d-amphetamine was shown to be a competitive inhibitor of the oxidation of TM. The Ki for d-amphetamine, as a MAO inhibitor, was lower in the ganglia than in the peripheral nerve endings.Supported by a Contract from the National Research Council of Argentina (CONICET) (Res. 67/79).     

Potentiation of the inhibition of xanthine oxidase by a combination of 6-mercaptopurine and 6-thioguanine

Summary Carcinostatic agents, 6-mercaptopurine and 6-thioguanine inhibited the in vitro and in vivo activity of the enzyme xanthine oxidase (xanthine-oxygen oxidoreductase E.C. 1.2.3.2.). Simultaneous, addition of a mixture of the 2 antimetabolites produced a synergistic effect on the inhibition of the enzyme activity.We thank the University Grants Commission, New Delhi, for their financial assistance and the Dean, M.G.M. Medical College Indore, for providing necessary facilities.     

‘Classical’ and ‘new’ diabetogens—comparison of their effects on isolated rat pancreatic islets in vitro

This study compares functional and morphological alterations caused by application of alloxan, streptozotocin, xanthine oxidase/hypoxanthine (generation of reactive oxygen species), or S-nitroso-N-acetyl-D,L-penicillamine (SNAP, liberation of nitric oxide) to isolated rat pancreatic islets in vitro. In perifusion experiments, membrane leakage—detected by non-stimulated insulin release—was found after application of all drugs, but showed a substance-specific time pattern. Twenty-four hours after application of the classical diabetogens (alloxan or streptozotocin), potassium chloride- and glucose-stimulated insulin secretion were markedly reduced, while a persistent reduction was observed neither after exposure to xanthine oxidase/hypoxanthine, nor to SNAP. Morphological analysis of the islets revealed that nearly all β-cells were destroyed following alloxan or streptozotocin treatment, while the majority of β-cells were configured regularly after application of xanthine oxidase/hypoxanthine or SNAP. Necrotic cells found after xanthine oxidase/hypoxanthine usually differed in morphology from those observed after application of the classical diabetogens. While the former cells were characterised by swollen nuclei, the latter had shrunken nuclei with irregular condensed chromatin. Apoptosis was found only following nitric oxide exposure. Due to these differences, it seems unlikely that alloxan, streptozotocin, xanthine oxidase/hypoxanthine, and nitric oxide have a common major feature in their toxic action. Received 16 September 1999; received after revision 15 November 1999; accepted 26 November 1999     

Assessment of the scavenging action of reduced glutathione, (+)-cyanidanol-3 and ethanol by the chemiluminescent response of the xanthine oxidase reaction

The free radical scavenging capacity of reduced glutathione (GSH), (+)-cyanidanol-3 and ethanol was assessed by their interference with the maximal chemiluminescent response produced by the xanthine oxidase reaction. GSH and (+)-cyanidanol-3 induce a progressive inhibition of chemiluminescence when increasing amounts are added to the reaction mixture. GSH and (+)-cyanidanol-3 added together at low concentrations (1 and 0.05 mM respectively) exhibit an additive effect. The addition of ethanol presents a biphasic effect. It inhibits chemiluminescence at low concentrations (10-50 mM) while at higher concentrations (75-500 mM) this effect is reversed. Estimation of the concentrations required to produce half of the maximal inhibition of chemiluminescence by these agents revealed that ethanol is less effective than GSH and (+)-cyanidanol-3 as a free radical scavenger in the system used.     

Occurrence of mitochondrial monoamine oxidase in human semen.

Mitochondrial monoamine oxidase (MAO) was found in human semen, showing its Km and Vmax values of 91.7 microM and 290 pmoles/mg of protein/60 min, respectively, with kynuramine as substrate. A major part of the activity was due to type A MAO.     

Involvement of xanthine oxidase and hypoxia-inducible factor 1 in Toll-like receptor 7/8-mediated activation of caspase 1 and interleukin-1β

Inflammatory reactions to ssRNA viruses are induced by the endosomal Toll-like receptors (TLRs) 7 and 8. TLR7/8-mediated inflammatory reaction results in activation of the Nalp3 inflammasome via an unknown mechanism. Here we report for the first time that TLR7/8 mediate activation of xanthine oxidase (XOD) in an HIF-1α-dependent manner. XOD produces uric acid and reactive oxygen species, which could activate Nalp3 and therefore induce activation of caspase 1, known to convert inactive pro-IL-1β into active IL-1β. Specific inhibition of the XOD activity attenuates TLR7/8-mediated activation of caspase 1 and IL-1β release. These results were obtained using human THP-1 myeloid macrophages. The findings were verified by conducting in vivo experiments on mice.     

Elevation of serum xanthine oxidase activity following halothane anesthesia in man

Summary Halothane, but not methoxyflurane, was found to cause specific hepatocellular damage, the hepatotoxicity being prompt but transient. The hepatotoxicity was demonstrated by the elevation in the serum activity of xanthine oxidase, a highly sensitive marker for acute liver damage.     

Elevation of serum xanthine oxidase following halothane anesthesia in the rat.

Halothan anesthesia was found to be hapatotoxic in the rat, as demonstrated by a significant elevation of serum xanthine oxidase (SXO) level. SXO appeared to be more sensitive marker of liver damage than serum glutamic oxalacetic transaminase. SXO was found to be elevated also following exposure to relative hypoxia.     

Elevation of serum xanthine oxidase following halothane anesthesia in the rat

Summary Halothane anesthesia was found to be hepatotoxic in the rat, as demonstrated by a significant elevation of serum xanthine oxidase (SXO) level. SXO appeared to be a more sensitive marker of liver damage than serum, glutamic oxalacetic transaminsa. SXO was found to be elevated also following exposure to relative hypoxia.     

A spectrophotometric method for the determination of 5-phosphoribosyl-1-pyrophosphate.

A new, sensitive, specific and simple spectrophotometric method for the determination of 5-phosphoribosyl-l-pyrophosphate (PRPP) is presented. PRPP is reacted with excess hypoxanthine in the presence of hypoxanthine-guanine phosphoribosyltransferase. At the end of the reaction, PRPP concentration is measured from the extent of conversion of hypoxanthine to inositate. The concentration of the purine base is determined spectrophotometrically in the presence of xanthine oxidase.     

A spectrophotometric method for the determination of 5-phosphoribosyl-1-pyrophosphate

Summary A new, sensitive, specific and simple spectrophotometric method for the determination of 5-phosphoribosyl-1-pyrophosphate (PRPP) is presented. PRPP is reacted with excess hypoxanthine in the presence of hypoxanthine-guanine phosphoribosyltransferase. At the end of the reaction, PRPP concentration is measured from the extent of conversion of hypoxanthine to inositate. The concentration of the purine base is determined spectrophotometrically in the presence of xanthine oxidase.     

Oxidation of synephrine by type A and type B monoamine oxidase

Summary Synephrine (SP) was found to be a substrate for monoamine oxidase (MAO) in rat brain mitochondria, showing the K_m and V_max values of 250 M and 32.6 nmoles/mg of protein/30 min respectively. The inhibition studies showed that the SP oxidation was carried out by both type A and type B MAO and a major part of the activity was due to type A MAO.     

Determination of the kinetic parameters of enzyme inhibition of the K-type mechanism: application to the control of alpha-glucosidase activity of honeybee hemolymph by glucose]

The alpha-glucosidasic activity of emerging honeybees haemolymph is submitted to a feed-back inhibition by glucose, according to a mechanism of the "K" type (competitive). The "resulting affinity-constant" (measured in the presence of the enzyme both with substrate and inhibitor) is linear function of the inhibitor concentration. The affinity constants between enzyme and pure substrate on one hand, and between enzyme and pure inhibitor on the other hand, were determined by means of this relation, which led to respectively equivalent values after determinations under in vitro or in vivo inhibitions.     

Chlorodimeform and its effect on monoamine oxidase activity in the cattle tick, Boophilus microplus.

The action of the acaricide, chlorodimeform and its metabolite. N-desmethylchlorodimeform, on the activity monoamine oxidase (MAO) from the cattle tick, Boophilus microplus, were studied. Both compounds were found to be potent in vitro and in vivo inhibitors of the enzyme. However the inhibition of MAO does not seem to be related to the toxic action of the acaricide.     

Chlorodimeform and its effect on monoamine oxidase activity in the cattle tickBoophilus microplus

Summary The action of the acaricide, chlorodimeform and its metabolite, N-desmethylchlorodimeform, on the activity monoamine oxidase (MAO) from the cattle tick,Boophilus microplus, were studied, Both compounds were found to be potent in vitro and in vivo inhibitors of the enzyme. However the inhibition of MAO does not seem to be related to the toxic action of the acaricide.     

SU5416 sensitizes ovarian cancer cells to cisplatin through inhibition of nucleotide excision repair

SU5416 is reported to be a selective inhibitor of vascular endothelial growth factor, and it has metwith limited success in the clinic. In the present study, we investigated whether SU5416 could augment cisplatin-induced cytotoxicity in human ovarian cancer cells. When used as a single agent, 2-h exposures to SU5416 were not harmful to the cells up to doses of 100 microM. For 48-h exposures, the SU5416 IC20 and IC50 were 17 and 34 microM, respectively. When used with cisplatin, the effect of SU5416 was sequence dependent. SU5416 given first was subadditive, whereas cisplatin given first was supraadditive. Cisplatin was given as a 1-h exposure. Augmented cisplatin cytotoxicity was seen with 2-h exposures to SU5416 at doses of 17-34 microM. This was associated with a decrease in cisplatin-DNA adduct repair, as measured by atomic absorbance spectrometry. Treatment of the ovarian carcinoma cells with SU5416 was also associated with a reduced expression of ERCC-1 protein and c-jun mRNA, as well as a decrease in c-Jun and JNK activities. We conclude that SU5416 can be used to augment cisplatin-induced cell killing at doses that are non-toxic. This effect may occur through direct or indirect reduction of the activity of AP-1 and DNA repair.     

MAO inhibition,an unlikely mode of action for chlordimeform

Summary Inhibition constants of several formamidines, their corresponding formanilides and other representatives of compounds derived from aniline, such as phenylureas, N-phenyl-carbamates and acylanilides, were determined for rat liver monoamine oxidase. The reversability of the inhibition and the lack of correlation between inhibition potencies and toxicities of the compounds tested add to the opinion that MAO inhibition is not a prominent factor in chlordimeform poisoning.     

Effect of cholesterol oxidation on (Na+, K+) ATPase activity of erythrocyte membranes.

By incubation of human erythrocyte ghosts with cholesterol oxidase (EC 1.1.3.6) part of the cholesterol of the membrane is replaced by 4-cholesten-3-one. This alteration in the sterol composition is accompanied by an inhibition of the (Na+, K+) ATPase of the erythrocyte membrane.     

Nitric oxide synthase reduces nitrite to NO under anoxia

Cultured bEND.3 endothelial cells show a marked increase in NO production when subjected to anoxia, even though the normal arginine pathway of NO formation is blocked due to absence of oxygen. The rate of anoxic NO production exceeds basal unstimulated NO synthesis in normoxic cells. The anoxic release of NO is mediated by endothelial nitric oxide synthase (eNOS), can be abolished by inhibitors of NOS and is accompanied by consumption of intracellular nitrite. The anoxic NO release is unaffected by the xanthine oxidase inhibitor oxypurinol. The phenomenon is attributed to anoxic reduction of intracellular nitrite by eNOS, and its magnitude and duration suggests that the nitrite reductase activity of eNOS is relevant for fast NO delivery in hypoxic vascular tissues. Received 20 August 2006; received after revision 21 September 2006; accepted 8 November 2006