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)

Abstract

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 V max values in microsomes and mitochondria being liver > lung > kidney and kidney > liver > lung, respectively. Although similar V max 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 V max values for radical formation of paraquat were higher than those of diquat in all the endogenous one-electron reducing systems. The apparent K m values for diquat, however, were lower than those for paraquat in both subcellular fractions from the ttree tissues, indicating the superiority of the reduction for diquat to that for paraquat at low concentrations. This difference in the K m 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 previossly proposed determines its toxicity.