Studies on bilirubin and steroid glucuronidation by rat liver microsomes

Abstract

The addition of digitonin or other detergents to liver microsomes from control and phenobarbital-treated rats increased the rate of glucuronidation of bilirubin, testosterone, estradiol, estrone and estriol. The extent of activation by digitonin as well as the concentration of detergent causing maximal activation was substrate dependent. Treatment of rats with sodium phenobarbital (75 mg/kg/day, i.p.) for 3 days significantly increased the glucuronidation of testosterone and estradiol per mg of protein but had no effect on estrone glucuronidation. Administration of 0.1% sodium phenobarbital in the drinking water for 20 days increased bilirubin, testosterone and estrone glucuronidation per mg of protein, but had little or no effect on estradiol and estriol glucuronidation. When UDP-glucuronyl transferase activity was calculated on a per g liver (wet wt) basis, however, an increase in the glucuronidation of estradiol and estriol was seen; this increase could be accounted for by a large stimulatory effect of phenobarbital on the synthesis of total microsomal protein. Liver microsomes from homozygous Gunn rats, which are deficient in the ability to conjugate bilirubin, did conjugate testosterone, estradiol. estrone and estriol, but some substrates were conjugated more rapidly than others. The addition ofdigitonin to liver microsomes from the homozygous Gunn rat increased the glucuronidation of testosterone but not that of estrone. Microsomal steroid glucuronidation in this animal was stimulated to varying degrees by the administration of sodium phenobarbital. The data suggest the presence of several glucuronyl transferases in liver microsomes.