Selective inhibition of bipyridyl-stimulated NADPH oxidation by ascorbic acid

Reduction in paraquat embryotoxicity by ascorbic acid in Xenopus laevis

The toxicity of herbicide Paraquat (PQ, 1-1'-dimethyl-4,4'bipyridylium dichloride) in animal cells is related to its rapid reduction and instantaneous reoxidation to produce the reactive oxygen species. Recently, the PQ evaluation with the Frog Embryo Teratogenesis Assay-Xenopus (FETAX) showed its high embryotoxicity. Supposing that the embryos' death was due to PQ-related oxidative damage, we used ascorbic acid (AA), a well known antioxidant, to reduce the PQ embryotoxicity in Xenopus laevis. Embryos were exposed from stage 8 to 47 to 0.1 mg/l PQ alone, and to PQ with AA concentrations ranging from 20 to 200 mg/l, using the FETAX procedure. PQ caused 72.2% mortality, while 17.1% of surviving larvae were affected by abnormal tail flexure. The PQ mortality percentages were reduced in a clear concentration-response by up to 15.2% in the group exposed to PQ with 200 mg/l AA. The histopathologic diagnoses revealed abnormal notochord flexure coupled with vesiculated, pear-shaped myocytes only in the PQ group. After embryo exposure to PQ with 200 mg/l AA, restoration of normal axial tail structures was evident. In conclusion, PQ embryotoxicity in X. laevis was most likely due to oxidative damage that was drastically reduced by AA.

Comparison of one-electron reduction activity against the bipyridylium herbicides, paraquat and diquat, in microsomal and mitochondrial fractions of liver, lung and kidney (in vitro)

The first one-electron reduction steps of paraquat and diquat were compared using microsomal and mitochondrial fractions of rat liver, lung and kidney. Both fractions reduced each herbicide effectively, with the order of the Vmax values in microsomes and mitochondria being liver greater than lung greater than kidney and kidney greater than liver greater than lung, respectively. Although similar Vmax values were obtained from the liver and lung with the two subcellular fractions, the affinity of mitochondrial enzymes was lower, suggesting that the reduction of both herbicides in a microsomal site would be dominant in these two organs. The Vmax values for radical formation of paraquat were higher than those of diquat in all the endogenous one-electron reducing systems. The apparent Km values for diquat, however, were lower than those for paraquat in both subcellular fractions from the three tissues, indicating the superiority of the reduction for diquat to that for paraquat at low concentrations. This difference in the Km values supported the finding that the reduction velocity for diquat was significantly higher than that for paraquat at 1 mM concentration. Thus, at low concentrations, diquat would be reduced more easily than paraquat. In addition, tissue enzymatic specificity for paraquat was not obtained. From these data, it seems reasonable to conclude that the tissue-selective accumulation of paraquat previously proposed determines its toxicity.