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Antoxidant Compounds

ANTIOXIDANT ACTIVITY OF PROPYLTHIOURACIL. Mark Hicks, Lisa Wong, and Richard O. Day Departmet of Clinical Pharmacology and Toxicology, St Vincent's Hospital, Victoria Street, Darlinghurst, NSW 2010, AUSTRALIA

Propylthiouracil, (PTU), has been shown to relieve a lcohol- induced hepatocyte damage i n v i t r o and was successful in clinical trials in ameliorating severe alcoholic liver disease. It has been postulated that one of the modes of action of PTU is as a scavenger of free radical products produced during metabolism of ethanol. The present study examines the reactions of PTU with

H O" and 0 2 radicals and its ability to directly inhibit peroxidation of lipids in a model membrane system. The rate constant for the reaction of PTU

with HO" derived from 7-radiolysis as determined by steady state compet i t ion kinet ics with

parani t rosodimethylani l ine was 8x109 M -1. s -1 PTU was less reactive towards superoxide, from either xanthine/xanthine oxidase or y - r a d i o l y s i s of formate. Linoleic acid vesicles, (final [linoleic acid]=5.SmM), were incubated with 0 to 200gM PTU

at 40°C. The rate of peroxidation was decreased from 0.072mM LOOH/hr to 0.024mM LOOH/hr in the presence of 0 to 50~tM PTU, however, this inhibitory effect was gradually lost as PTU concentrations increased above 50gM. Addition of micromolar desferr ioxamine and PTU inhibited peroxidation in an additive fashion. These results indicate that PTU is both an efficient hydroxyl radical scavenger and antioxidant at therapeutic levels of free drug (around 50gM).

~ A N T I ~ ~ ~ MAIARIA Nicholas H. Hunt, *Roland Stocker, Martin Kopp and Shane B. T~, Department of Pathology, University of Sydney and *Heart Research Institute, C ~ , N~R, /K~-'tralia.

There is substantial evidence consistent with the idea that phagocyte-derived oxidants are involved in Plasmodia killing and tissue damage in malaria infection. However, direct evidence for o~idati~ damage to parasites cr host organs during the natural course of the infection is sparse. The redox status of the parasitized erythrocyte (PRBC) is shifted ~ reduction (Stock~r et al. H~A 876, 294, 1986), which r~=H~ to he reconciled with the notiGn that the PRBC is exposed to oxidative stress. We therefore examined total plasma glutathione content and GSH:GSSG ratio in murine malaria infection. Total glutathicne was significantly decreased in mice heavily parasitized with P.vinck~i and the GSH:C~SG ratio was increased. Thus, export of GSSG from PRBC is unlikely to explain the i r ~ celln]a~ redox ratio. I ~ plas~ uric acid and ascorbate are seen in murine malaria (Stock~r et al. ~ 1__3, 152, 1986). We now have demonstrated (by HPLC) progressive increases in bilirubin levels in plasma during the course of infection. Overall, the anti- oxidant capacity of plasma seems to be enhanced in mJrine n-alaria ard t h i s c ~ l d serve t o ~lial cells and unparasitized erythr~ against phagocyte-derived oxidants.

DIPYRIDAMOLE INHIBITS LIPID PEROXIDATION AT TWO DIFFERENT LEVELS: THE INITIATION REACTION AND THE PROPAGATION CHAIN REACTION• L• luliano D. Pratico' A. Ghiselli C. Piccheri F. Violi nst tute of _a C n ca Med ca, Un versity "La SSpienza",

Rome, Italy.

We have previously reported that Dipyridamole a well nown antiplatelet drug, possesses anti~xidant

activity, The drug inhibits polyunsaturated fa,',ty acid perox=oation induced by iron salts. Herewith we report further studies on the mechanism or antioxioant activity of dipyridamole. Spectroscopic studies, did not reyealed any chelating activity or uipyrioamole. Furthermore two stages of lipid p.eroxidation were investigated: the initiation phase, dependent on OH ° radicals, and the propagation chain reaction, dependent on the presence of-LeO ° radical. The O1-1 ~. dependent initiation of lipid p.eroxidation was executed on X-irradiation of water oispersed micelles of arachidonic acid at a dose rate of 11.3 Gy/min. In this system Dipyridamole inhibited arachidonic acid peroxidation, detected by MDA formation, in a dose dependent manner. The propagation chain reaction was stuc_Jied by. peroxidizing arachidonic acid with per.o.xyl radical generateo oy thermolysis or azo-Dis-amioinopropane and the p.eroxidation m. onitored..by a. Clarktype oxygen e~ectrooe. Again u~pynoamo~e ~nn~o~teo arac-hidonic acid perox=dation in a dose dependent manner. These data confirm the antioxidant activity of dipyridamole and give insight in this biochemical mechanism ,by showing that the d.rug acts at two different levels: the initiation step and the propagat ion chain reaction,

OXIDANT STRESS DURING ACETAMINOPHEN-INDUCED HEPATOTOXlClTY: PROTECTION BY ALLOPURINOL

Hartmut Jaeschke. Department of Medicine, Baylor College of Medicine, Houston, Texas 77030, U.S.A.

The role of an intracellular oxidant stress in the pathogenesis of acetaminophen (AAP)-induced hepatotoxicity remained controversial since no GSSG formation was detected in vivo (CBI 70:241, 1989). AAP (500 mg/kg) caused a Ionglasting 50% reduction of the hepatic ATP content in male ICR mice, an irreversible loss of hepatic xanthine dehydrogenase activity and a transient increase of the xanthine oxidase activity. All effects occured prior to parenchymal cell damage, i.e. the release of cellular enzymes. The hepatic content of GSH and GSSG was initially depleted by AAP without affecting the GSSG-to-GSH ratio (1:200), however, during the recovery phase of the hepatic glutathione levels the GSSG content increased faster than GSH resulting in a GSSG-to-GSH ratio of 1:18 24 h after AAP administration. The mitochondrial GSSG content increased from 2% in controls to >20% in acetaminophen-treated mice. The extremely elevated tissue GSSG levels were accompanied by a fourfold increase of the plasma GSSG concentrations but not by an enhanced biliary efflux, although hepatic GSSG formation and biliary excretion were not affected by acetaminophen. AIIopurinol protected dose-dependently against AAP-induced cell injury, the loss of ATP and the increase of tissue GSSG without inhibiting reactive metabolite formation. High, protective as well as low, nonprotective doses of allopurinol almost completely inhibited hepatic xanthine oxidase and dehydrogenase activity. The data indicate a Ionglasting, primarily intracellular oxidant stress during the progression phase of acetaminophen-lnduced cell necrosis. The protective effect of allopurinol is unlikely to involve the inhibition of reactive oxygen formation by xanthine oxidase but could be the result of its antioxidant property.

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