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Publisher Summary The reactions catalyzed by citratase and isocitratase are reversible retrograde aldol reactions of the corresponding six-carbon tricarboxylic acids yielding, in each case, a two-carbon carboxylic acid and a four-carbon dicarboxylic acid. The chapter describes the assay method, purification procedure, and properties of citratase and isocitratase (isocitric lyase). Citratase activity is conveniently measured by the formation of oxalacetate with citrate as substrate. Two methods have been applied: (1) manometric, with extracts where oxalacetate decarboxylase may be present; and (2) spectrophotometric, for oxalacetate accumulation when the decarboxylase is absent. Citratase formation, activity, and some degree of purification is studied in lactic acid bacteria. Isocitratase activity is measured most readily with isocitrate as substrate, left to right above, by determining glyoxylate accumulation with carbonyl reagents as (1) the 2,4-dinitrophenylhydrazone 3° by a slight modification of the Friedemann and Haugen pyruvate determination and as (2) the semicarbazone. Isocitratase has been purified from Pseudomonas aeruginosa, ATCC 9027, about fiftyfold and from baker's yeast about seventyfold. |
