Characteristics of a microsomal cytochrome P-448-mediated reaction. Ethoxyresorufin O-de-ethylation.
Certain characteristics of ethoxyresorufin O-de-ethylation, as catalyzed by microsomes of liver, lung, and intestine of control and pretreated rats and hamsters, were studied. The results support previous suggestions that the reaction is catalyzed primarily by a 3-methyl-cholanthrene (MC)-inducible mono-oxygenase which has a 448-nm absorption maximum in the reduced-CO difference spectrum. Ethoxyresorufin exhibited a type I binding spectrum with liver microsomes from MC-induced rats, but there was no clear interaction with microsomes from control or phenobarbital (PB)-induced rats. Maximum MC-induction of type I binding and de-ethylase activity coincided with the appearance of a cytochrome P-450 spectrum whose absorption maximum was shifted to 448 nm. Low concentrations of alpha-naphthoflavone (ANF) or benzo[a]pyrene (BP) inhibited the de-ethylation with liver microsomes of MC-treated rats (150 approximately 10(-9)M) but not those of control rats. A kinetic analysis of BP inhibition of this reaction showed it to be competitive. Inhibition of the MC-induced liver microsomal reaction by low concentrations of BP or ANF diminished rapidly with time. MC-induced rat liver microsomal de-ethylation of ethoxyresorufin was less sensitive than the PB-induced reaction to inhibition by metyrapone or SKF 525-A (I50 approximately 10(-6) - 10(-4)M). However, microsomes from rat liver, lung, and intestine had very low constitutive activities (less than 0.1 nmol/min/mg of protein). MC greatly induced the de-ethylation reaction in liver (200 X), intestine (40 X) and lung (10 X). De-ethylation of ethoxyresorufin in microsomes from control and MC-induced rat lung was inhibited by low concentrations of either ANF or BP (I50 approximately 10(-8) M). Control hamster liver microsomes were several times more active in de-ethylation than control rat liver microsomes, but MC-induction of hamster liver was only 1/10 of that in rat liver. Control hamster lung activity was similar to that of control rat lung, but was not appreciably induced by MC.
D000818:Animals; D006224:Cricetinae; D003580:Cytochromes / Q000378:metabolism / Q000502:physiology*; D003640:Dealkylation; D004790:Enzyme Induction; D005470:Fluorometry; D066298:In Vitro Techniques; D007421:Intestine, Small / Q000201:enzymology / Q000648:ultrastructure; D008168:Lung / Q000201:enzymology / Q000648:ultrastructure; D008297:Male; D008647:Mesocricetus; D008748:Methylcholanthrene / Q000494:pharmacology; D008861:Microsomes / Q000201:enzymology*; D008862:Microsomes, Liver / Q000201:enzymology; D010078:Oxazines / Q000378:metabolism*; D010105:Oxygenases / Q000037:antagonists & inhibitors / Q000378:metabolism; D051381:Rats; D013053:Spectrophotometry; D013997:Time Factors
D003580:Cytochromes; D010078:Oxazines; D008748:Methylcholanthrene; C007649:ethoxyresorufin; D010105:Oxygenases