S-adenosylhomocysteinase from mouse liver. Catalytic properties at cellular enzyme level

Measurement of intracellular sulfur amino acid metabolism in humans

Methionine metabolism forms homocysteine via transmethylation. Homocysteine is either 1) condensed to form cystathionine, which is cleaved to form cysteine, or 2) remethylated back to methionine. Measuring this cycle with the use of isotopically labeled methionine tracers is problematic, because the tracer is infused into and measured from blood, whereas methionine metabolism occurs inside cells. Because plasma homocysteine and cystathionine arise from intracellular metabolism of methionine, plasma homocysteine and cystathionine enrichments can be used to define intracellular methionine enrichment during an infusion of labeled methionine. Eight healthy, postabsorptive volunteers were given a primed continuous infusion of [1-13C]methionine and [methyl-2H(3)]methionine for 8 h. Enrichments in plasma methionine, [13C]homocysteine and [13C]cystathionine were measured. In contrast to plasma methionine enrichments, the plasma [13C]homocysteine and [13C]cystathionine enrichments rose to plateau slowly (rate constant: 0.40 +/- 0.03 and 0.49 +/- 0.09 h(-1), respectively). The enrichment ratios of plasma [13C]homocysteine to [13C]methionine and [13C]cystathionine to [13C]methionine were 58 +/- 3 and 54 +/- 3%, respectively, demonstrating a large intracellular/extracellular partitioning of methionine. These values were used to correct methionine kinetics. The corrections increase previously reported rates of methionine kinetics by approximately 40%.

Formation of S-adenosylhomocysteine in the heart. I: An index of free intracellular adenosine

To assess the concentration of free intracellular adenosine in the heart the kinetic properties of cytosolic S-adenosylhomocysteine (SAH) hydrolase were utilized at elevated levels of L-homocysteine (adenosine + L-homocysteine in equilibrium with SAH + H2O). Global hypoxia was induced in the isolated perfused guinea pig heart by graded reduction of perfusion medium PO2 in the presence of saturating concentrations of homocysteine (0.2-1.0 mM). Reduction of PO2 from 660 to 165 mm Hg increased the steady-state concentration of total tissue adenosine from 2.0 +/- 0.2 to 2.8 +/- 0.2 nmoles/g, while the rate of SAH formation increased linearly from 0.22 +/- 0.03 to 2.50 +/- 0.13 nmoles/min/g. When adenosine was exogenously applied at a concentration of 100 microM together with homocysteine (1 mM), SAH accumulation rates were much greater: 23.34 +/- 3.31 and 42.11 +/- 1.73 nmoles/min/g with normoxic (95% O2) and hypoxic (30% O2) perfusion, respectively. The apparent Km and Vmax values for SAH-hydrolase in vivo were estimated to be 20 microM and 59 nmoles/min/g wet wt, respectively. Since the relation between SAH formation and adenosine in the physiological concentration range is linear, the measured rate of SAH accumulation during normoxia and hypoxia permitted the calculation of the free intracellular adenosine level, which was 0.061 nmoles/g (0.08 microM) in the normoxic heart. With hypoxia (PO2 165 mm Hg), this value increased to 1.57 nmoles/g (2.0 microM). Free intracellular adenosine closely correlated with the hypoxia-induced changes in coronary flow. The data reveal that measurement of the rate of SAH accumulation during homocysteine infusion can be used for sensitive assessment of free intracellular adenosine levels. Assuming that the intracellular adenosine concentration equals that in the interstitial space, the results furthermore indicate that the degree of intracellular adenosine formation during hypoxic perfusion is quantitatively sufficient to account for most of the observed increases in coronary flow.

Binding of S-adenosylhomocysteine to hydroxyindole O-methyltransferase

Biochim Biophys Acta 1984 May 31;787(1 ) 1-7 (S-adenosyl-L-methionine:N-acetylserotonin O-methyltransferase, EC 2.1.1.4) purified from bovine pineal gland forms a complex with S-adenosylhomocysteine, one of the products of the reaction catalyzed by the enzyme. The binding of S-adenosylhomocysteine to the enzyme has been characterized by the use of S-[8-14C]adenosylhomocysteine or S-[U -14C]adenosylhomocysteine. The complex did not dissociate during filtration through Sephadex G-25. Long-term dialysis against ligand-free phosphate buffer did bot dissociate the bound S-adenosylhomocysteine. S-Adenosylhomocysteine co-migrated with hydroxyindole O-methyltransferase on disc polyacrylamide gel electrophoresis. Although the complex was stable even under nonequilibrium conditions, the bound S- adenosylhomocysteine was separated into adenosine and homocysteine in the presence of S- adenosylhomocysteine hydrolase. The binding of S-adenosylhomocysteine was optimal at pH 7.0, but was not dependent on temperature. Scatchard plots showed that Kd was 6.5 X 10(-9) M and the maximal binding was 1 mol of S-adenosylhomocysteine per subunit of the enzyme. Hydroxyindole O-methyltransferase cannot form a stable complex with S-adenosylmethionine, and the addition of excess amounts of S-adenosylmethionine impairs the binding of S-adenosylhomocysteine to the enzyme. The product inhibition by S-adenosylhomocysteine may be based on the binding of S-adenosylhomocysteine to the enzyme with high affinity and on the stable enzyme-product complex formation during the transmethylation reaction.

S-Adenosylhomocysteinase from yellow lupin seeds: stoichiometry and reactions of the enzyme-adenosine complex

Plant (Lupinus luteus) S-adenosylhomocysteinase, an alpha 2 dimer, forms a 1:2 enzyme-adenosine complex. The binding sites for adenosine are not equivalent. Binding of the first molecule of adenosine is fast (k greater than 7 x 10(5) M-1 s-1), whereas the second molecule of adenosine binds in a slow process with a half-life of 5 min. Adenosine in the 1:1 and 1:2 enzyme--substrate complexes reacts slowly (k = 0.05 min-1) to give finally free enzyme, adenine, and ribose. The enzyme does not lose its ability to catalyze the synthesis of S-adenosylhomocysteine during the reactions. The relevance of the data to the catalytic functioning of the plant S-adenosylhomocysteinase is discussed.