title: A possible role of inorganic phosphate as a regulator of oxidative phosphorylation in combined urea synthesis and gluconeogenesis in perfused rat liver. A phosphorus magnetic resonance spectroscopy study.
abstract: Metabolic control of oxidative metabolism was studied in perfused rat liver by means of phosphorus magnetic resonance spectroscopy. Oxygen consumption, ATP, and Pi were measured with different rates of gluconeogenesis and urea synthesis by varying concentrations of the substrates in the perfusate. Five levels of oxygen consumption (VO2) were obtained: an average control value of 1.94 +/- 0.14 and 2.93 +/- 0.25, 3.29 +/- 0.46, 3.85 +/- 0.26, and 4.18 +/- 0.56 mumol/min/g liver (mean +/- S.D., n = 6). The corresponding ATP concentrations were 2.51 +/- 0.20, 2.39 +/- 0.08, 2.24 +/- 0.09, 2.13 +/- 0.12, and 1.91 +/- 0.13 mM. Pi increased stoichiometrically with the decrease in ATP. Free Pi (Pif) was calculated as NMR-visible Pi in control plus -delta ATP (1.94 mM + (-delta ATP]. The kinetic relationship of oxidative phosphorylation as a function of Pif followed a Michaelis-Menten type of equation: VO2 = 5.55/(1 + 0.24/[( Pif] - 1.81]. The observed Km value for Pi of 0.24 mM approximates the reported Km value in isolated mitochondria of 1 mM. The free Pi concentration of 1.94 mM is in the range of the Km value, while the free ADP concentration of 200 microM exceeds the Km value of 20 microM. Therefore, it is suggested that Pi play a major role in the regulation of mitochondrial oxidative phosphorylation in combined urea synthesis and gluconeogenesis.
authors.affiliation: Department of Biochemistry and Biophysics, University of Pennsylvania, Philadelphia 19104.
authors.forename: A
authors.initials: A
authors.lastname: Tanaka
authors.forename: B
authors.initials: B
authors.lastname: Chance
authors.forename: B
authors.initials: B
authors.lastname: Quistorff
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D000643:Ammonium Chloride / Q000494:pharmacology; D000818:Animals; D005943:Gluconeogenesis / Q000187:drug effects; D006706:Homeostasis; D008099:Liver / Q000187:drug effects / Q000378:metabolism*; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D008297:Male; D010085:Oxidative Phosphorylation / Q000187:drug effects; D010477:Perfusion; D010710:Phosphates / Q000378:metabolism*; D010758:Phosphorus; D051381:Rats; D011919:Rats, Inbred Strains; D012016:Reference Values; D014508:Urea / Q000096:biosynthesis*
chemical_list: D010710:Phosphates; D000643:Ammonium Chloride; D010758:Phosphorus; D000255:Adenosine Triphosphate; D014508:Urea