title: Degradation of cytochrome P-450 haem by carbon tetrachloride and 2-allyl-2-isopropylacetamide in rat liver in vivo and in vitro. Involvement of non-carbon monoxide-forming mechanisms.
abstract: Degradation of intrinsic hepatic [(14)C]haem was analysed as (14)CO formation in living rats and in hepatic microsomal fractions prepared from these animals 16h after pulse-labelling with 5-amino[5-(14)C]laevulinic acid, a precursor that labels bridge carbons of haem in non-erythroid tissues. NADPH-catalysed peroxidation of microsomal lipids in vitro (measured as malondialdehyde) was accompanied by loss of cytochrome P-450 and microsome-associated [(14)C]haem (largely cytochrome P-450 haem), but little (14)CO formation. No additional (14)CO was formed when carbon tetrachloride and 2-allyl-2-isopropylacetamide were added to stimulate lipid peroxidation and increase loss of cytochrome P-450 [(14)C]haem. Because the latter effect persisted despite inhibition of lipid peroxidation with MnCl(2) or phenyl-t-butylnitrone(a spin-trapping agent for free radicals), it was concluded that carbon tetrachloride, as reported for 2-allyl-2-isopropylacetamide, may promote loss of cytochrome P-450 haem through a non-CO-forming mechanism independent of lipid peroxidation. By comparison with breakdown of intrinsic haem, catabolism of [(14)C]methaemalbumin by microsomal haem oxygenase in vitro produced equimolar quantities of (14)CO and bilirubin, although these catabolites reflected only 18% of the degraded [(14)C]haem. This value was increased to 100% by addition of MnCl(2), which suggests that lipid peroxidation may be involved in degradation of exogenous haem to products other than CO. Phenyl-t-butylnitrone completely blocked haem oxygenase activity, which suggests that hydroxy free radicals may represent a species of active oxygen used by this enzyme system. After administration of carbon tetrachloride or 2-allyl-2-isopropylacetamide to labelled rats, hepatic [(14)C]haem was decreased and haem oxygenase activity was unchanged; however, (14)CO excretion was either unchanged (carbon tetrachloride) or decreased (2-allyl-2-isopropylacetamide). These changes were unaffected by cycloheximide pretreatment. From the lack of parallel losses of cytochrome P-450 [(14)C]haem and (14)CO excretion, one may infer that an important fraction of hepatic [(14)C]haem in normal rats is degraded by endogenous pathways not involving CO. We conclude that carbon tetrachloride and 2-allyl-2-isopropylacetamide accelerate catabolism of cytochrome P-450 haem through mechanisms that do not yield CO as an end product, and that are insensitive to cycloheximide and independent of haem oxygenase activity.
mesh_terms: D000081:Acetamides / Q000494:pharmacology*; D000502:Allylisopropylacetamide / Q000494:pharmacology*; D000622:Aminolevulinic Acid / Q000378:metabolism; D000818:Animals; D002248:Carbon Monoxide / Q000096:biosynthesis; D002251:Carbon Tetrachloride / Q000494:pharmacology*; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D006418:Heme / Q000378:metabolism*; D006419:Heme Oxygenase (Decyclizing) / Q000378:metabolism; D066298:In Vitro Techniques; D007668:Kidney / Q000187:drug effects / Q000378:metabolism; D050356:Lipid Metabolism; D008099:Liver / Q000187:drug effects / Q000378:metabolism*; D008297:Male; D008862:Microsomes, Liver / Q000187:drug effects / Q000378:metabolism; D051381:Rats
chemical_list: D000081:Acetamides; D000502:Allylisopropylacetamide; D006418:Heme; D002248:Carbon Monoxide; D000622:Aminolevulinic Acid; D003577:Cytochrome P-450 Enzyme System; D002251:Carbon Tetrachloride; D006419:Heme Oxygenase (Decyclizing)