Inhibition of hepatic fatty acid oxidation at carnitine palmitoyltransferase I by the peroxisome proliferator 2-hydroxy-3-propyl-4-[6-(tetrazol-5-yl) hexyloxy]acetophenone.
Recent studies suggest that the induction of peroxisomal beta-oxidation in rodents may represent an adaptive response to disturbances in hepatic lipid metabolism. The following studies were done to determine the effects of 2-hydroxy-3-propyl-4-[6-(tetrazol-5-yl)hexyloxy]acetophenone (4-THA), a tetrazole-substituted acetophenone which induces peroxisomal beta-oxidation in rodent liver, on fatty acid oxidation in vitro. In isolated hepatocytes, 4-THA inhibited the oxidation of oleate (C18:1) and decreased the mitochondrial redox state. The inhibition was more pronounced in the presence of 0.2 mM-oleate than with 0.5 mM, indicating the inhibition may be competitive. 4-THA had no effect on the oxidation of octanoate (C8:0), suggesting that the site of inhibition of oleate oxidation was the carnitine-dependent transport across the mitochondrial inner membrane. In rat liver mitochondria, 4-THA inhibited carnitine palmitoyltransferase I (CPT-I) competitively with respect to the substrate palmitoyl-CoA, increasing the apparent Km from 19 microM to 86 microM. The inhibition of CPT-I by 4-THA was independent of the concentration of the co-substrate carnitine. Whereas fasting attenuated the inhibition of CPT-I by malonyl-CoA, it did not diminish the inhibition by 4-THA. Inhibition of transferase activity by 4-THA and malonyl-CoA was attenuated in mitochondria which had been solubilized with octyl glucoside to expose the latent form of carnitine palmitoyltransferase (CPT-II), suggesting that the inhibition was specific for CPT-I. The specificity was further demonstrated in studies of mitochondrial beta-oxidation in which 4-THA inhibited the oxidation of palmitoyl-CoA but not palmitoylcarnitine. The results demonstrate that 4-THA inhibits fatty acid oxidation in rat liver in vitro at the site of transport across the mitochondrial inner membrane, CPT-I. Whether this disruption in mitochondrial oxidation is causally related to the induction of peroxisomal beta-oxidation is yet to be determined.
Lilly Research Laboratories, Biochemical Toxicology Department, Greenfield, IN 46140.
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Foxworthy
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D000098:Acetophenones / Q000494:pharmacology*; D000217:Acyltransferases / Q000037:antagonists & inhibitors*; D000818:Animals; D001393:Azoles / Q000494:pharmacology*; D002210:Caprylates / Q000378:metabolism; D002331:Carnitine / Q000494:pharmacology; D002334:Carnitine O-Palmitoyltransferase / Q000037:antagonists & inhibitors*; D005227:Fatty Acids / Q000378:metabolism*; D066298:In Vitro Techniques; D007527:Isoenzymes / Q000037:antagonists & inhibitors*; D007657:Ketone Bodies / Q000096:biosynthesis; D008099:Liver / Q000187:drug effects / Q000378:metabolism*; D008297:Male; D008830:Microbodies / Q000378:metabolism; D019301:Oleic Acid; D009829:Oleic Acids / Q000378:metabolism; D010084:Oxidation-Reduction; D010171:Palmitoyl Coenzyme A / Q000378:metabolism; D010172:Palmitoylcarnitine / Q000378:metabolism; D051381:Rats; D011916:Rats, Inbred F344; D013777:Tetrazoles / Q000494:pharmacology*
D000098:Acetophenones; D001393:Azoles; D002210:Caprylates; D005227:Fatty Acids; D007527:Isoenzymes; D007657:Ketone Bodies; D009829:Oleic Acids; D013777:Tetrazoles; D010171:Palmitoyl Coenzyme A; D010172:Palmitoylcarnitine; D019301:Oleic Acid; C056958:2-hydroxy-3-propyl-4-(6-(tetrazol-5-yl)hexyloxy)acetophenone; D000217:Acyltransferases; D002334:Carnitine O-Palmitoyltransferase; C031492:octanoic acid; D002331:Carnitine