The Influence of Glucuronidation on in Vitro Assessment of Bilirubin Production as Measure of HO Activity

Heme oxygenase (HO) catalyzes the rate-limiting step in the degradation of heme to bilirubin. Glucuronidation of the cytotoxic bilirubin is a critical step in heme catabolism. HO and UDP-glucuronyl transferase (UGT) enzymes are co-localized in the membranes of the endoplasmic reticulum allowing efficient glucuronidation of bilirubin. Besides its physiological significance, this co-localization has practical consequences as microsomal protein is used to assess HO activity in biological samples. In the present study the influence of sample specific glucuronidation capacity on the production of chloroform-extractable bilirubin was investigated. Addition ofexogenous UDP-glucuronic acid significantly decreased the levels of measurable bilirubin. In contrast, dialysis of the tissue-cytosol fraction, which is routinely added to the mixture as source of biliverdin reductase, completely eliminated glucuronidation activity of the HO mixture and significantly increased the concentration of chloroform-extractable bilirubin. Thus, varying levels of UGT-activity in the individual samples may greatly influence the assessment of microsomal HO activity.