Adenosine phosphate ratios

In living organisms, the adenosine phosphates are in equilibrium and are regarded collectively as the adenylic acid system. The physiological concentrations of ADP and ATP are around 10 mol/1. The ratio of the forms is called the energy charge (EC), and... 

Brain hexokinase is inhibited by its product glucose-6-phosphate and to a lesser extent by adenosine diphosphate. The isoenzyme of hexokinase found in brain may be soluble in the cytosol or be attached firmly to mitochondria [2 and references therein]. An equilibrium exists between the soluble and the bound enzyme. The binding changes the kinetic properties of hexokinase and its inhibition by Glc-6-P resulting in a more active enzyme. The extent of binding is inversely related to the ATP ADP ratio, i.e. conditions in which energy utilization... 

Figure 22.17 Summary of mechanisms to maintain the ATP/ADP concentration ratio in hypoxic myocardium. A decrease in the ATP/ADP concentration ratio increases the concentrations of AMP and phosphate, which stimulate conversion of glycogen/ glucose to lactic acid and hence ATP generation from glycolysis. The changes also increase the activity of AMP deaminase, which increases the formation and hence the concentration of adenosine. The latter has two major effects, (i) It relaxes smooth muscle in the arterioles, which results in vasodilation that provides more oxygen for aerobic ATP generation (oxidative phosphorylation). (ii) It results in decreased work by the heart (i.e. decrease in contractile activity), (mechanisms given in the text) which decreases ATP utilisation.

The nucleotide anhydride, adenosine 5 -triphosphate (24), when digested with aqueous barium hydroxide, gives a complex mixture containing such products as adenine, adenosine, adenosine 2 -, 3 -, and 5 -phosphates, adenosine 5 -pyrophosphate, and adenosine 2 (or 3 ),5 -diphosphate. - In addition, a nucleotide was foimd in this digest whose structure proved - to be that of adenosine 3 5 -cyclic phosphate (25). This component did not consume metaperiodate, and was degraded enzymically to adenosine 5 -phosphate (26) and adenosine 3 -phosphate (27), without the formation of adenosine 2 -phosphate. Hydrolysis of (25) with an acidic ion-exchange resin did, however, produce the 2 - and 3 -phosphates of adenosine. Compound (25) possessed only one phosphoryl dissociation, and showed a ratio of nucleoside to phosphate of 1 1, which, along with a molecular-... 

For lipogenesis, glucose 6-phosphate is converted through glycolysis to pyruvate. Key enzymes that regulate this pathway in the liver are phosphofructokinase-1 (PFK-1) and pyruvate kinase. PFK-1 is aliosterically activated in the fed state by fructose 2,6-bisphosphate and adenosine monophosphate (AMP) (see Fig. 36.1). Phosphofructokinase-2, the enzyme that produces the activator fructose 2,6-bisphosphate, is dephosphorylated and active after a meal (see Chapter 22). Pyruvate kinase is also activated by dephosphorylation, which is stimulated by the increase of the insulin/glucagon ratio in the fed state (see Fig. 36.1). 

The supernatant liquor (S-100 solution) from the centrifugation of disrupted cells of this resistant strain at lOO.OOOg was obtained by a procedure similar to that already described for kanamydn-neomycin phosphate transferase I (see p. 186). Kana-mycin A was inactivated in a reaction mixture that consisted of the following materials in the following volume ratios S-100 solution (10 mg of protein per ml), 4 vols. a ten-times concentrated buffer solution (0.6M potassium chloride, 0.1 M magnesium acetate, 0.06 M 2-mercaptoethanol, 1.0 M Tris hydrochloride, pH 7.8), 2 vols. 165 mM adenosine 5 -triphosphate, 1 vol. 0.08 M creatine phosphate, 0.5 vol. 1.2 mg/ml of creatine kinase, 0.5 vol. 1.2 mM coenzyme A, 1 vol. and the solution" of the antibiotic, 1 vol. The inactivated kanamycin was extracted and purified by chromatography on a column of silicic acid, followed by chromatography on Dowex-1 X-2 (OH ) resin. 

The adenosine kinases from animal cells have broad specificities with respect to their substrates. Thus, ATP, ITP, and GTP can all serve as phosphate donors, althoi h to varying degrees. The nucleoside triphosphate is probably involved as the Mg + complex, and the Mg +-nucleoside triphosphate ratio is critical. The Michaelis constant for ATP is about 5-10 X 10 M, depending on the Mg + concentration. The molecular weight of the rabbit liver enzyme is about 233,000. 

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