Microsomal UDP-glucuronyltransferase in rat liver : Oxidative activation

• Published in: Basic & clinical pharmacology & toxicology 2005-06, Vol.96 (6), p.480-486
• Main Authors: LETELIER, Maria Eugenia, PIMENTEL, Alejandra, PINO, Patricia, LEPE, Ana Maria, FAUNDEZ, Mario, ARACENA, Paula, SPEISKY, Hernan
• Format: Article
• Language: English
• Subjects: Animals; Ascorbic Acid - pharmacology; Biological and medical sciences; Cysteine - pharmacology; Dithiothreitol - pharmacology; Enzyme Activation; Glucuronosyltransferase - metabolism; Glutathione - pharmacology; In Vitro Techniques; Iron - pharmacology; Lipid Peroxidation - drug effects; Male; Medical sciences; Microsomes, Liver - drug effects; Microsomes, Liver - enzymology; Microsomes, Liver - metabolism; Octoxynol - pharmacology; Oxidation-Reduction; Pharmacology. Drug treatments; Rats; Rats, Sprague-Dawley; Sulfhydryl Compounds - metabolism
• ISSN: 1742-7835; 1742-7843
• DOI: 10.1111/j.1742-7843.2005.pto_12.x

Activation of microsomal UDP-glucuronyltransferase (UDPGT) activity by treatment of hepatic microsomes with either detergents or Fe(3+)/ascorbate pro-oxidant system has been reported; however, definite mechanisms underlying these effects have not been clarified. In this work, we characterize Fe(3+)/ascorbate-induced activation of UDPGT activity prior to solubilization with Triton X-100 and after the oxidation process provoked the solubilization of the enzyme. We observed a time-dependent increase in UDPGT activity up to 20 min. incubation of the microsomes with Fe(3+)/ascorbate (3-times); after 20 min. incubation, however, we observed a time-dependent decrease in this activity to basal levels after 4 hr incubation. Treatment of microsomes with 0.1% Triton X-100 (5 min.) lead to a similar increase in UDPGT activity; higher detergent concentrations produced a dose-dependent decrease in this activity to basal levels with 1% Triton X-100. Interestingly, UDPGT activity was susceptible to activation only when associated to microsomal membranes and the loss of activation correlated with the solubilization of this activity. UDPGT activation by either Fe(3+)/ascorbate or Triton X-100 was correlated with an increase in p-nitrophenol apparent K(m) and V(max) values. This activation was prevented or reversed by the reducing agents glutathione, cysteine or dithiothreitol when it was induced by the Fe(3+)/ascorbate. Furthermore, the latter provoked a significant decrease in microsomal thiol content, effect not observed after treatment with Triton X-100. Our results suggest that the main mechanism responsible for Fe(3+)/ascorbate-induced UDPGT activation is likely to be the promotion of protein sulfhydryl oxidation; this mechanism appears to be different from detergent-induced UDPGT activation.