DIFFERENTIAL EXPRESSION OF HEPATIC UDP-GLUCURONOSYLTRANSFERASE DURINGDIETARY MAGNESIUM DEPLETION

Male Sprague Dawley rats weighing 80+/-5 g were fed either a magnesium deficient (MgD) diet (30, 66 or 85 mg/kg) or a pair-fed control (PFC) diet (595 mg/kg) for 10 days. Serum Mg'' was decreased 66% in the mag nesium depleted animals (on diets of 30 and 66 mg of Mg+2/kg), no sign ificant change was noted for total liver magnesium concentration. Hepa tic microsmes were prepared and used fresh or stored at -70-degrees-C. The total UDP-glucuronosyl-transferase (UDP-GT) activities were evalu ated using microsomes activated with Triton X-100. Three isoenzyme act ivities of UDP-GT were determined for specific substrates: p-nitrophen ol (pNP), bilirubin (BR) and lithocholate (LiC). Addition of magnesium (10 mM) to the in vitro reaction system increased each isoenzyme acti vity, pNP UDP-GT (100%), BR UDP-GT (350%) and LiC UDP-GT (100%). The s pecific effect of dietary magnesium depletion was a decrease in pNP UD P-GT activity while the BR UDP-GT and LiC UDP-GT isoenzyme activities were not significantly changed. Below a dietary intake of about 2 mg o f magnesium/day, the loss of pNP UDP-GT isoenzyme activity was proport ional to the decrease in dietary Mg+2 intake. Thus, expression of the pNP UDP-GT was specifically diminished at early stages of magnesium de pletion. The mechanism of this selective Mg+2 effect is under further investigation.