Influence of para Substituent of Phenol on Phospholipid Dependence of Uridine Diphosphate-glucuronyltransferase

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The influence of para substituent of phenol on the property of phospholipid dependence of uridine diphosphate-glucuronyltransferase (UDPGT) was studied using hepatic microsomes of the rats. para Substituents used were p-ethyl, p-tert-butyl and p-phenyl. Glucuronidations of phenol and its derivatives were inhibited with each other. When phospholipids of the microsomes were removed with phospholipase A_2,V_<max> and K_m decreased in the order phenol, p-ethylphenol, p-tert-butylphenol, p-phenylphenol, indicating that this change by delipidation increased with van der Waals volume (V_w) of para substituent. Arrhenius plots of glucuronidation for phenol and p-ethylphenol exhibited a change in slope, responding to thermotropic property of the membrane labelled with 1,6-diphenyl-1,3,5-hexatriene (DPH). In contrast, p-tert-butyl and p-phenyl substituents showed linear Arrhenius plots. The activation energy (E_a) for glucuronidation of p-phenylphenol was increased by delipidation, whereas E_a of other substrates did not show any remarkable change.
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