Comparisons of flux control exerted by mitochondrial outer-membrane carnitine palmitoyltransferase over ketogenesis in hepatocytes and mitochondria isolated from suckling or adult rats.
The primary aim of this paper was to calculate and report flux control coefficients for mitochondrial outer-membrane carnitine palmitoyltransferase (CPT I) over hepatic ketogenesis because its role in controlling this pathway during the neonatal period is of academic importance and immediate clinical relevance. Using hepatocytes isolated from suckling rats as our model system, we measured CPT I activity and carbon flux from palmitate to ketone bodies and to CO2 in the absence and presence of a range of concentrations of etomoxir. (This is converted in situ to etomoxir-CoA which is a specific inhibitor of the enzyme.) From these data we calculated the individual flux control coefficients for CPT I over ketogenesis, CO2 production and total carbon flux (0.51 +/- 0.03; -1.30 +/- 0.26; 0.55 +/- 0.07, respectively) and compared them with equivalent coefficients calculated by similar analyses [Drynan, L., Quant, P.A. & Zammit, V.A. (1996) Biochem. J. 317, 791-795] in hepatocytes isolated from adult rats (0.85 +/- 0.20; 0.23 +/- 0.06; 1.06 +/- 0.29). CPT I exerts significantly less control over ketogenesis in hepatocytes isolated from suckling rats than those from adult rats. In the suckling systems the flux control coefficients for CPT I over ketogenesis specifically and over total carbon flux (< 0.6) are not consistent with the enzyme being rate-limiting. Broadly similar results were obtained and conclusions drawn by reanalysis of previous data {from experiments in mitochondria isolated from suckling or adult rats [Krauss, S., Lascelles, C.V., Zammit, V.A. & Quant, P.A. (1996) Biochem. J. 319, 427-433]} using a different approach of control analysis, although it is not strictly valid to compare flux control coefficients from different systems. Our overall conclusion is that flux control coefficients for CPT I over oxidative fluxes from palmitate (or palmitoyl-CoA) differ markedly according to (a) the metabolic state, (b) the stage of development, (c) the specific pathway studied and (d) the model system.
New
K J
KJ
Unit of Paediatric Surgery, University College Medical School, London, UK.
Krauss
S
S
Elliott
K R
KR
Quant
P A
PA
D000818:Animals; D000833:Animals, Suckling; D002245:Carbon Dioxide / Q000378:metabolism; D002331:Carnitine / Q000378:metabolism; D002334:Carnitine O-Palmitoyltransferase / Q000378:metabolism*; D002478:Cells, Cultured; D004734:Energy Metabolism; D004791:Enzyme Inhibitors / Q000494:pharmacology; D004852:Epoxy Compounds / Q000494:pharmacology; D007657:Ketone Bodies / Q000378:metabolism*; D008099:Liver / Q000378:metabolism*; D008316:Malonyl Coenzyme A / Q000378:metabolism; D008565:Membrane Proteins / Q000378:metabolism*; D008930:Mitochondria, Liver / Q000201:enzymology*; D019308:Palmitic Acid / Q000378:metabolism; D010171:Palmitoyl Coenzyme A / Q000378:metabolism; D010172:Palmitoylcarnitine / Q000378:metabolism; D051381:Rats; D017208:Rats, Wistar
D004791:Enzyme Inhibitors; D004852:Epoxy Compounds; D007657:Ketone Bodies; D008565:Membrane Proteins; D002245:Carbon Dioxide; D010171:Palmitoyl Coenzyme A; D010172:Palmitoylcarnitine; D019308:Palmitic Acid; D008316:Malonyl Coenzyme A; D002334:Carnitine O-Palmitoyltransferase; C054207:etomoxir; D002331:Carnitine