Evidence against cytochrome b5 involvement in liver microsomal fatty acid elongation.
This study provides strong evidence against cytochrome b5 participation in the first reduction step-beta-ketoreduction-of rat liver microsomal fatty acid chain elongation. Several lines of evidence led to this conclusion: (a) beta-ketoreductase was not inducible by diet conditions since its activity was the same in microsomes from fasted rats and in rats fed a fat-free diet. Consequently, its activity was appreciable in microsomes from fasted rats. Nevertheless, cytochrome b5 reoxidation rate was not stimulated by adding beta-ketopalmitoyl-CoA to the latter microsomes. This suggests that it is not the activated beta-ketoreductase which stimulates the cytochrome b5 reoxidation rate, but another electron acceptor. (b) The delta 9-desaturase, present in microsomes from rats fed a fat-free diet, was totally inhibited by 4 mM KCN; beta-ketopalmitoyl-CoA or malonyl-CoA stimulated the reoxidation rate of cytochrome b5 but this increase was also inhibited by 4 mM KCN. This suggests that delta 9-desaturase is involved in the stimulation and shows that any inhibitor of delta 9-desaturase, including cytochrome b5 antibodies, may induce elongation inhibition. (c) NADH-dependent beta-ketoreductase activity was partially purified from Triton X-100 solubilised microsomes, in a fraction essentially free of cytochrome b5. Furthermore, when the fraction containing cytochrome b5 and NADH-cytochrome-b5 reductase was added to the fraction containing beta-ketoreductase activity, no increase in beta-ketoreductase activity was observed. Stearoyl-CoA desaturase activity which is also present in microsomes from rats fed a fat-free diet led to the results which have been misinterpreted in the conclusions of previous studies.
Demirkapi
N
N
Institut National de la Santé et de la Recherche Médicale, U 56, Hôpital de Bicêtre, Le Kremlin-Bicêtre, France.
Carreau
J P
JP
Ghesquier
D
D
D000429:Alcohol Oxidoreductases / Q000378:metabolism; D000818:Animals; D015786:Cytochromes b5 / Q000378:metabolism*; D004041:Dietary Fats / Q000008:administration & dosage; D005215:Fasting; D044943:Fatty Acid Desaturases / Q000037:antagonists & inhibitors / Q000378:metabolism; D005227:Fatty Acids / Q000378:metabolism*; D008297:Male; D008316:Malonyl Coenzyme A / Q000494:pharmacology; D008862:Microsomes, Liver / Q000187:drug effects / Q000201:enzymology*; D009243:NAD / Q000494:pharmacology; D009249:NADP / Q000494:pharmacology; D010084:Oxidation-Reduction; D010171:Palmitoyl Coenzyme A / Q000494:pharmacology; D011190:Potassium Cyanide / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D013230:Stearoyl-CoA Desaturase
D004041:Dietary Fats; D005227:Fatty Acids; D009243:NAD; D010171:Palmitoyl Coenzyme A; C040149:beta-ketohexadecanoyl-coenzyme A; D008316:Malonyl Coenzyme A; D009249:NADP; D015786:Cytochromes b5; D000429:Alcohol Oxidoreductases; C023744:acetoacetyl-CoA reductase; D044943:Fatty Acid Desaturases; D013230:Stearoyl-CoA Desaturase; C029627:delta-9 fatty acid desaturase; D011190:Potassium Cyanide