Add time:08/27/2019         Source:sciencedirect.com

Publisher SummaryThis chapter elaborates a method for the enzymatic determination of acetoacetyl coenzyme A. The method is based on the principle that β-hydroxyacyl dehydrogenase (HOADH) catalyzes the reaction in the fatty acid cycle as described in the chapter. The reaction is pyridine nucleotide-dependent and therefore can be measured spectrophotometrically. The equilibrium constant K is 1.9 × 10–10 moles/l, at 25°C. The position of the equilibrium allows quantitative reduction of acetoacetyl-CoA with reduced diphosphopyridine nucleotide (DPNH) at pH values up to about 7. The decrease in absorption at 340 mμ because of the oxidation of DPNH serves as a measure of the reaction. Solutions of β-hydroxyacyl dehydrogenase are stable for long periods at 0°C; repeated freezing and thawing lead to a rapid decrease in activity. Solutions of acetoacetyl-CoA should be between pH 4 and 6; they are stable for several months in the frozen state. Acetoacetyl-CoA is hydrolyzed by an alkali just as rapidly as acetyl-CoA and like the latter it is sensitive to the prolonged action of strong acid. The presence of high concentrations of mercaptans, especially cysteine, should be avoided at pH values greater than 6.5 (thiol exchange).

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