The effect of paraquat on the in vitro activity of cytosol, mitochondrial and microsomal enzyme systems.

Subcellular fractions (mitochondria, microsomes and cytosol) were prepared from the lungs of rabbits and rats to investigate the effects of paraquat (Aldrich Laboratories) on the activity of some cytosol and mitochondrial dehydrogenases and on the microsomal respiration and reduced pyridine nucleotide oxidation rate. The normal basal oxygen consumption of rabbit lung microsomes was 1,9 +/- 0,3 nmol O2/mg microsomal protein/min, and the oxidation rates of reduced nicotinamide-adenine dinucleotide phosphate (NADPH) and reduced nicotinamide-adenine dinucleotide (NADH) were 4,29 +/- 0,53 and 4,0 +/- 0,55 nmol/mg microsomal protein/min respectively. One molecule of oxygen can therefore oxidize two molecules of NADPH or NADH, and the generated hydrogen peroxide is probably immediately broken down by the catalase activity of the normal lung microsomal preparation. When Aldrich paraquat (1,0 mM) was added to microsomes metabolizing NADPH (0,5-0,75 mM), both the rate of oxygen consumption and the generation of nicotinamide-adenine dinucleotide phosphate (NADP) were significantly (P less than 0,001) stimulated over the first 5 minutes, and thereafter returned to within basal limits. When microsomes were preincubated with 1,0 mM paraquat before NADPH was added, the oxygen consumption was substantially lower (10,01 +/- 1,01 nmol oxygen/mg microsomal protein/min), while the NADPH oxidation rate was almost similar to the basal rate in the absence of paraquat. This resulted in a striking dissociation in the H/O ratio under these circumstances. The addition of potassium cyanide (KCN) (5.0 mM) prior to paraquat pre-incubation and followed by the addition of NADPH restored the stimulatory effect of paraquat on microsomal respiration and on NADPH oxidation rate.(ABSTRACT TRUNCATED AT 250 WORDS)