ABSTRACT:Studies were performed to investigate the UDP-glucuronosyltransferase enzyme (

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The substituent dependencies of the first (ß = -0.09 ± 0.02) and second (ß^= -0.38 ± 0.03) ionization constants have been measured for some aryl monophosphate esters at 25 °C and 1M ionic strength. The Bronsted selectivity (0eq = -1-35 ± 0.06) has been deduced from the literature data for the equilibrium constants for the hydrolysis of monophosphate dianions covering a wide range of oxygen donor acidity. Assuming that the Bronsted values for ionization follow similar selectivities as in the aryl esters we may deduce the /3eq exponents for transfer of phosphoryl (phosphonate, -P032"), phosphono monoanion (-P03H_), and phosphono (-P03H2) groups of-1.36, -1.74, and -1.83, respectively. The Bronsted selectivity data indicate that the phosphono (-P03H2) group has approximately the same electropositivity as the acetyl function; successive ionization reduces the electropositivity of the phosphyl group. Knowledge of the /?eq values provides standards with which to compare the effective charge changes on the oxygen donor in monophosphate dianion and monoanion hydrolyses.

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