The effects of graded doses of paraquat, a widely used herbicide, on t
he activities of antioxidant defense and detoxifying enzymes in the lu
ng and the liver of rats were studied in adult male Fischer 344 rats.
Paraquat was administered by intraperitoneal (i.p.) injection to 7 gro
ups of rats at the dose of 0, 5, 10, 20, 30, 40 or 50 mg/kg body weigh
t (BW), respectively. The rats were killed 24 hours after injection, a
nd the lungs and the livers were excised for biochemical assays. The r
esults showed that the effects of paraquat on antioxidant defense enzy
mes were different in the two tissues. In the lung, paraquat increased
the activities of catalase, GPx, GST and GR at lower doses, but moder
ately depressed those of CuZnSOD, GPx, GR and severely depressed gluta
thione-S-transferase (GST) activity at the highest dose. In the liver,
paraquat slightly depressed the activities of glutathione peroxidase
(GPx) and GST, severely depressed catalase activity, but it did not af
fect the activities of CuZn superoxide dismutase (SOD), MnSOD and glut
athione reductase(GR). The results suggest that paraquat administratio
n in lower doses may induce a protective mechanism of endogenous antio
xidant defenses in the target organ, the lung, against oxidative stres
s caused by paraquat. However, at the highest dose most of the major a
ntioxidant defense enzymes were moderately depressed and GST was sever
ely depressed, suggesting that the antioxidant defense and detoxifying
capability of the lung was compromized. The results also suggest that
the depression of antioxidant defense and detoxification capability m
ay contribute to the toxicity of paraquat.