Imidazole Catalyses in Aqueous Systems. VIII. Spontaneous Hydrolysis and Michaelis-Menten-type Catalytic Hydrolysis of Phenyl Esters in the Presence of Cholic Acid and Its Histamine Derivative

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Spontaneous hydrolyses of cationic and anionic phenyl esters in the presence of cholic acid (VI) and their catalytic hydrolyses with a histamine amide of cholic acid (VII) were investigated using a pH-stat at 30°C in 1.0M aqueous KCl. The rates of spontaneous hydrolysis decreased by 30–70% upon binding of substrates with cholic acid. The variation of the apparent binding constant (7-3650M<SUP>−1</SUP>) was considered to reflect the magnitude of hydrophobic forces between cholic acid and substrates, although the values may have to be corrected for the possible involvement of self-association of cholic acid molecules. Imidazole derivative VII catalyzed the hydrolysis of phenyl esters according to Michaelis-Menten kinetics. The long methylene chain in the substrate molecule contributed appreciably to the binding of substrate with cholic acid and its derivative. The magnitude of substrate binding with VII appears to be comparable to the true binding constant with cholic acid. The intracomplex rate constants did not differ much from those expected with other enzyme-like catalysts. Thus, the steroid ring seems to provide a non-specific hydrophobic binding site for the substrates used.
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