Effects of silymarin on UDP-glucuronic acid and glucuronidation activity in the rat isolated hepatocytes and liver in relation to D-galactosamine toxicity.

Influence of silymarin on UDP-glucuronic acid (UDPGA) and glucuronidation activity of freshly isolated rat hepatocytes in suspension and in rat liver in vivo was examined. Viability of the hepatocytes (> 85%) was not altered in Hank's balanced salt solution at least for 4 hr at 37 degrees C under oxygen. Silymarin at 0.4 mM depleted UDPGA by more than 60% at the end of 4 hr of incubation, the fall in nucleotide pool was rapid and concentration (0.1-0.4 mM)-dependent. The rate of glucuronidation of 3-OH- benzo(a)pyrene (3-OH-BP) determined simultaneously was also reduced significantly; silybin being 3-times more effective than silymarin. Combination of flavonoids with D-galactosamine (GalN) further attenuated the glucuronidation functions of the cells. The flavonoids also offered strong inhibition of UDP-glucose dehydrogenase (UDP-GDH) activity in the liver cytosolic fraction while the activity in hepatocytes was not affected even after 4 hr of incubation. Interestingly, the GalN- induced strong inhibition of UDP-GDH in isolated hepatocytes was completely abolished by flavonoids. Decrease in UDPGA appeared neither due to the activation of UDPGA-pyrophosphatase activity nor to the inhibition of UDP-GDH activity in hepatocytes. Further, the flavonoids also inhibited hepatic UDP-glucuronyltransferase activity towards 3-OH-BP (UGT) both in vitro and in intact cells. On the contrary, silymarin administered (70 mg/kg body wt; i.p.) to rats for 3 hr increased the hepatic UDPGA by 2-fold while GalN (400 mg/kg body wt) reduced the nucleotide content to 50% of control. Coadministration of silymarin and GalN restored the UDPGA content significantly while the activities of UDP-GDH and UGT were comparable to the untreated control. The results indicated that silymarin elicits differential effects on the rate of glucuronidation and contents of UDPGA in the isolated rat hepatocytes and in liver. The flavonoid counteracted D-GalN-induced lowering of UDPGA presumably by relieving UDP-GDH of in vivo inhibition affected by GalN-metabolite.

Influence of uridine diphosphoglucuronic acid on bilirubin glucuronidation in liver microsomes from normal and spironolactone treated rats

En el presente trabajo se estudió la formación de glucuronoconjugados de bilirrubina por microsomas hepáticos de ratas normales y tratadas con el inductor espironolactona (SP). A los efectos de analizar la influencia del ácido uridina difosfoglucurónico (UDPGA) sobre dicho proceso se incorporaron cantidades crecientes de éste a las mezclas de incubación. Los microsomas de ratas tratadas con SP mostraron mayor capacidad de formación de glucurónidos de bilirrubina que los correspondientes testigos, fundamentalmente a expensas del diglucurónido (BDG). Por otro lado, al utilizar concentraciones crecientes de UDPGA en los medios de incubación, aumentó la formación de BDG en mayor proporción que la del monoglucurónido. Doicho comportamiento apoya la hipótesis de la existencia de diferentes subunidades enzimáticas glucuronizantes, con diferentes sespecificidades de sustrato. Por su parte, SP representa un efectivo inductor de la glucuronoconjugación de la bilirrubina, corroborando lo observado previamente en el animal entero