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 coef ficients for mitochondrial outer-membrane carnitine palmitoyltransferase (C PT I) over hepatic ketogenesis because its role in controlling this pathway during the neonatal period is of academic importance and immediate clinica l relevance. Using hepatocytes isolated from suckling rats as our model sys tem, we measured CPT I activity and carbon flux from palmitate to ketone bo dies 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 i nhibitor of the enzyme.) From these data we calculated the individual flux control coefficients for CPT I over ketogenesis, CO2 production and total c arbon flux (0.51 +/- 0.03; - 1.30 +/- 0.26; 0.55 +/- 0.07, respectively) an d compared them with equivalent coefficients calculated by similar analyses [Drynan, L., Quant, P.A. & Zammit, V.A. (1996) Biochem. J. 317, 791-795] i n 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 hepa tocytes isolated from suckling rats than those from adult rats. In the suck ling systems the flux control coefficients for CPT I over ketogenesis speci fically and over total carbon flux (< 0.6) are not consistent with the enzy me being rate-limiting. Broadly similar results were obtained and conclusio ns drawn by reanalysis of previous data (from experiments in mitochondria i solated from suckling or adult rats [Krauss, S., Lascelles, C.V., Zammit, V .A, gr Quant, P.A. (1996) Biochem. J. 319, 427-433]} using a different appr oach of control analysis, although it is not strictly valid to compare flux contol 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 mod el system.