ABSTRACT
The effect of natural "activation factor" and synthetic fructose-2,6-P2 on the allosteric kinetic properties of liver and muscle phosphofructokinases was investigated. Both synthetic and natural fructose-2,6-P2 show identical effects on the allosteric kinetic properties of both enzymes. Fructose-2,6-P2 counteracts inhibition by ATP and citrate and decreases the Km for fructose-6-P. This fructose ester also acts synergistically with AMP in releasing ATP inhibition. The Km values of liver and muscle phosphofructokinase for fructose-2,6-P2 in the presence of 1.25 mM ATP are 12 milliunits/ml (or 24 nM) and 5 milliunits/ml (or 10 nM), respectively. At near physiological concentrations of ATP (3 mM) and fructose-6-P (0.2 mM), however, the Km values for fructose-2,6-P2 are increased to 12 microM and 0.8 microM for liver and muscle enzymes, respectively. Thus, fructose-2,6-P2 is the most potent activator of the enzyme compared to other known activators such as fructose-1,6-P2. The rates of the reaction catalyzed by the enzymes under the above conditions are nonlinear: the rates decelerate in the absence or in the presence of lower concentrations of fructose-2,6-P2, but the rates become linear in the presence of higher concentrations of fructose-2,6-P2. Fructose-2,6-P2 also protects phosphofructokinase against inactivation by heat. Fructose-2,6-P2, therefore, may be the most important allosteric effector in regulation of phosphofructokinase in liver as well as in other tissues.
Subject(s)
Fructosediphosphates/pharmacology , Hexosediphosphates/pharmacology , Liver/enzymology , Muscles/enzymology , Phosphofructokinase-1/metabolism , Adenosine Monophosphate/pharmacology , Adenosine Triphosphate/metabolism , Allosteric Regulation , Animals , Citrates/metabolism , Enzyme Activation , Kinetics , Organ Specificity , RatsSubject(s)
Fructose-Bisphosphatase/pharmacology , Muscles/enzymology , Phosphofructokinase-1/metabolism , Adenosine Triphosphate/pharmacology , Allosteric Regulation , Anilino Naphthalenesulfonates , Animals , Carbon Radioisotopes , Chickens , Drug Synergism , Fructose-Bisphosphatase/metabolism , Fructosephosphates/isolation & purification , Fructosephosphates/metabolism , Gluconeogenesis , Glycolysis , Hydrogen-Ion Concentration , Kinetics , Liver/enzymology , Molecular Weight , Phosphofructokinase-1/antagonists & inhibitors , Phosphorus Radioisotopes , Protein Binding , Protein Conformation , Rabbits , Spectrometry, Fluorescence , Structure-Activity Relationship , UltracentrifugationABSTRACT
Studies were performed to examine synthesis, tissue localization, and metabolism of mevalonic acid in normal rats. Circulating mevalonate was found to have a rapid turnover phase of 5 min and a slower phase of 40-50 min. Under in vitro conditions the synthesis of mevalonate is carried out most actively by the liver and only to a minor extent by the other tissues studied. The most unexpected finding of this study was that both in vivo and in vitro the kidneys rather than the liver are the primary site of the metabolism of circulating mevalonate. Whereas mevalonate in the liver is rapidly transformed to cholesterol, the major products of mevalonate metabolism in the renal tissues during the same time period are squalene and lanosterol. Exogenous in contrast to circulating mevalonate is metabolized primarily in the intestine.
