J. Biochem, 1984, Vol. 96, No. 5 1323-1336
© 1984 Japanese Biochemical Society
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Role of Cytosolic Superoxide Dismutase as a Stimulator in Anthranilamide Hydroxylation by a Microsomal Monooxygenase System in Rat Liver
*Department of Biochemistry, School of Medicine Toyoake, Aichi 470–11
**Department of Biochemistry, School of Hygiene Toyoake, Aichi 470–11
***Department of Biochemistry, College of Medical Technology, Fujita-Gakuen Health University Toyoake, Aichi 470–11
We have isolated a protein factor from rat liver which stimulates anthranilamide hydroxylation by the microsomes in the presence of NADPH and oxygen and showed this factor to contain Cu and Zn and to have superoxide dismutase activity [Biochim. Biophys. Acta 365, 148–157 (1974)]. In the present study, this protein factor was confirmed to be a superoxide dismutase (SOD) by comparison of the recovery of SOD activity with that of anthranilamide hydroxylation-stimulating activity at each step of its purification, by inhibition of SOD activity with NaCN and hydrogen peroxide (H2O2), and by recovery of the SOD activity of the protein factor after reconstitution with Cu2+ and/or Zn2+. At a given SOD activity level, there was no difference among the rat liver SOD, Cu, Zn-SOD from bovine erythrocytes, and Mn-SOD from Serratia marcescens in their ability to stimulate anthranilamide hydroxylation not only by rat liver microsomes, but also by the reconstituted cytochrome P-450-containing monooxygenase system. Rat liver SOD stimulated anthranilamide hydroxylation by the reconstituted system in proportion to its amount below a protein concentration of 1 µg/ml. In anthranilamide hydroxylation by the reconstituted system without SOD, only a slight hydroxylase activity was found at the initial stage of the reaction and a marked increase in the amounts of NADPH oxidized and H2O2 formed was observed after a lag time. In the presence of rat liver SOD, however, the hydroxylase activity was markedly and continuously increased almost proportionally to reaction time with a concomitant decrease in the amounts of NADPH oxidized and H2O2 formed. In addition, a trace of 3-OH anthranilamide, one of the products, not only stimulated NADPH-dependent H2O2 formation in the reconstituted system, but also inhibited the apparent reduction of cytochrome P-450 by NADPH in the reconstituted system. These effects of 3-OH anthranilamide were diminished by rat liver SOD. When a trace of 3-OH anthranilamide were added to a system composed of NADPH-cytochrome c (P-450) reductase and NADPH, H2O2 formation and NADPH oxidation were markedly stimulated. However, on addition of 3-OH anthranilamide to the system containing rat liver SOD, no stimulation on either H2O2 formation or NADPH oxidation was found.
From these results, we propose the following mechanism for the stimulatory action of rat liver SOD on anthranilamide hydroxylation by the liver microsomal monooxygenase system: In anthranilamide hydroxylation by the monooxygenase, the SOD causes an increase in the amount of electrons flowing from NADPH to cytochrome P-450 by depressing an intensive NADPH-dependent H2O2 formation induced by 3-OH anthranilamide via NADPH-cytochrome c reductase, resulting in an enhancement of the efficiency of hydroxylation of the substrate by cytochrome c P-450.
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