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Purification and Properties of an Enzyme in Human Blood and Rat Liver Microsomes Catalyzing the Formation and Hydrolysis of -Lactones

II. METAL ION EFFECTS, KINETICS, AND EQUILIBRIA

From the ¹ From the University of Maryland Medical School, Baltimore, Maryland 21201

Calcium ions have been found to activate and stabilize -lactonase specifically. Hydrolytic activity at pH 8.6 requires 10 µm calcium ions, while the lactonization rate at pH 6.0 is maximal with 10 mm calcium. At pH 6.0, hydrolysis is increased proportionally with synthesis upon the addition of calcium, indicating that the metal ion interacts with the enzyme and not the substrate. Calcium ion, 1 mm, serves to stabilize -lactonase prepared from rat liver microsomes. Zn⁺⁺, Cu⁺⁺, Mn⁺⁺, Mg⁺⁺, Fe⁺⁺⁺, Ni⁺⁺, Co⁺⁺, Hg⁺⁺, Na⁺, and K⁺ either inhibit -lactonase or are without effect, depending upon the pH of the assay and the particular metal ion involved. Magnesium appears to inhibit specifically only the hydrolytic activity of -lactonase without affecting its lactonizing function. Ethylenediaminetetraacetate-calcium chelates can replace calcium in activation of the enzyme, indicating a very high affinity constant for the lactonase-calcium complex.

Four- to eight-carbon lactones are effective substrates for the enzymes, with K_m values ranging from 1 to 13 mm. The values for the corresponding -hydroxy acids as substrates for lactone synthesis run about 4-fold higher. The enzyme preparations from liver microsomes and human plasma have similar, but not identical, kinetic properties. Equilibrium constants at near neutral pH values and 27° have been determined for the 4- to 8-carbon lactone-hydroxy acid couples, with the use of -lactonase. Except for the 4-carbon substrate, a sizeable fraction of these compounds exists in the lactone form at equilibrium (10 to 20%).

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