Effect of combined treatment with 2,3,7,8-tetrachlorodibenzo-p-dioxin and phototherapy on bilirubin metabolism in the jaundiced Gunn rat

Cytochrome P450 1A2 Is Incapable of Oxidizing Bilirubin Under Physiological Conditions

Background: Bilirubin (BR) is metabolized mainly by uridine diphosphate (UDP)-glucuronosyltransferase 1A1 (UGT1A1) through glucuronidation in the liver. Some studies have shown that several subtypes of cytochrome P450 (CYP) enzymes, including CYP1A2, are upregulated by inducers and proposed to be alternative BR degradation enzymes. However, no information is available on the BR degradation ability of CYP in normal rats without manipulation by CYP inducers.

Methods: Quantitative real-time polymerase chain reaction (QRT-PCR), western blot, immunofluorescence, and confocal microscopy were used to find expression of CYP1A2 in the brain and the liver. BR metabolites in microsomal fractions during development were examined by high-performance liquid chromatography/electrospray ionization tandem mass spectrometry (LC-MS/MS).

Results: In the present study, we observed that CYP1A2 mRNA levels increased at postnatal days (P)14 and P30 with respect to the level at P7 both in liver and brain, this increment was especially pronounced in the brain at P14. The expression of CYP1A2 in the brainstem (BS) was higher than that in the cerebellum (CLL) and cortex (COR). Meanwhile, the CYP1A2 protein level was significantly higher in the COR than in the brainstem and CLL at P14. The levels of BR and its metabolites (m/z values 301, 315, 333 and biliverdin) were statistically unaltered by incubation with liver and brain microsomal fractions.

Conclusion: Our results indicated that the region-specific expression of CYP1A2 increased during development, but CYP family enzymes were physiologically incapable of metabolizing BR. The ability of CYPs to oxidize BR may be triggered by CYP inducers.

Inducible bilirubin oxidase: a novel function for the mouse cytochrome P450 2A5

We have previously shown that bilirubin (BR), a breakdown product of haem, is a strong inhibitor and a high affinity substrate of the mouse cytochrome P450 2A5 (CYP2A5). The antioxidant BR, which is cytotoxic at high concentrations, is potentially useful in cellular protection against oxygen radicals if its intracellular levels can be strictly controlled. The mechanisms that regulate cellular BR levels are still obscure. In this paper we provide preliminary evidence for a novel function of CYP2A5 as hepatic "BR oxidase". A high-performance liquid chromatography/electrospray ionisation mass spectrometry screening showed that recombinant yeast microsomes expressing the CYP2A5 oxidise BR to biliverdin, as the main metabolite, and to three other smaller products with m/z values of 301, 315 and 333. The metabolic profile is significantly different from that of chemical oxidation of BR. In chemical oxidation the smaller products were the main metabolites. This suggests that the enzymatic reaction is selective, towards biliverdin production. Bilirubin treatment of primary hepatocytes increased the CYP2A5 protein and activity levels with no effect on the corresponding mRNA. Co-treatment with cycloheximide (CHX), a protein synthesis inhibitor, resulted in increased half-life of the CYP2A5 compared to cells treated only with CHX. Collectively, the observations suggest that the CYP2A5 is potentially an inducible "BR oxidase" where BR may accelerate its own metabolism through stabilization of the CYP2A5 protein. It is possible that this metabolic pathway is potentially part of the machinery controlling intracellular BR levels in transient oxidative stress situations, in which high amounts of BR are produced.

CYP1A in TCDD toxicity and in physiology-with particular reference to CYP dependent arachidonic acid metabolism and other endogenous substrates

Toxicologic and physiologic roles of CYP1A enzyme induction, the major biochemical effect of aryl hydrocarbon receptor activation by TCDD and other receptor ligands, are unknown. Evidence is presented that CYP1A exerts biologic effects via metabolism of endogenous substrates (i.e., arachidonic acid, other eicosanoids, estrogens, bilirubin, and melatonin), production of reactive oxygen, and effects on K(+) and Ca(2+) channels. These interrelated pathways may connect CYP1A induction to TCDD toxicities, including cardiotoxicity, vascular dysfunction, and wasting. They may also underlie homeostatic roles for CYP1A, especially when transiently induced by common chemical exposures and environmental conditions (i.e., tryptophan photoproducts, dietary indoles, and changes in oxygen tension).

Novel kinetic insights into treatment of unconjugated hyperbilirubinemia: phototherapy and orlistat treatment in Gunn rats

Treatment with phototherapy or with the lipase inhibitor orlistat decreases plasma unconjugated bilirubin (UCB) concentrations in hyperbilirubinemic Gunn rats. We investigated the mechanism(s) underlying the effects of orlistat, phototherapy, and combined treatment, using steady-state 3H-UCB kinetics. After three weeks of treatment with orlistat (200 mg/kg chow), phototherapy (19 microW/cm2/nm) or combined treatment, tracer 3H-UCB was administered IV to treated and untreated (control) Gunn rats. Plasma samples and feces were collected every 12h for 60h, and bile for 30 min at 60h. The following results were obtained: 1) each treatment decreased plasma bilirubin levels compared with controls: orlistat- 19%, phototherapy-32%, combined treatment-53%; 2) plasma bilirubin concentrations were strongly, negatively correlated with fractional bilirubin turnover; 3) orlistat treatment induced net transmucosal excretion of UCB into the intestinal lumen, whereas phototherapy increased biliary UCB excretion rate; 4) all treatments profoundly increased the enterohepatic circulation of UCB derivatives, indicating enhanced metabolism by intestinal bacteria. In conclusion, orlistat and phototherapy lower plasma bilirubin concentrations in Gunn rats by increasing (net) intestinal influx of UCB, either by transmucosal excretion (orlistat), or increased biliary secretion (phototherapy). The mechanism of phototherapy and orlistat treatment involves increasing the availability of UCB in the intestinal lumen for fecal excretion and for metabolism by intestinal bacteria.

High-performance liquid chromatographic method for simultaneous determination of [1-methyl-14C]caffeine and its eight major metabolites in rat urine

A selective and sensitive reversed-phase liquid chromatographic method was developed for the simultaneous analysis of [1-Me-14C]caffeine and its eight major radiolabelled metabolites in rat urine. The separation of the complex mixture of caffeine metabolites was achieved by gradient elution with a dual solvent system using an endcapped C18 reversed-phase column, which in contrast to commonly used C18 reversed-phase columns also allows the separation of the two isomers of 6-amino-5-(N-formylmethylamino)-1,3-dimethyluracil (1,3,7-DAU), a caffeine metabolite of quantitative importance predominantly occurring in rat. As caffeine is metabolised primarily by members of the cytochrome P450 1A (CYP1A) subfamiliy, determination of the pattern of caffeine metabolites in rat urine enables analysis of activities of this important enzyme subfamily in vivo. Since CYP1A is suggested to be involved in the detoxification of bilirubin, the assay may be applied to search for untoxic inducers of CYP1A which might be of pharmacological interest in the treatment of hyperbilirubinaemia.

Transepithelial fate of bilirubin in the isolated rat kidney

The renal handling of bilirubin in the rat was studied using an isolated kidney preparation by means of the determination of total pigment concentration decay in the perfusion medium and its renal clearance. Unconjugated bilirubin was incorporated in the perfusate at a concentration of about 4 microg/ml. In order to establish the potential role of secretion in renal handling of the pigment, experiments were also performed incorporating in the perfusate different doses of nicotinic acid (NA) (0.1 and 1.0 mM final concentration), which is considered an alternative substrate for the organic anion transport system, or probenecid (Prob) (0.1 and 1.0 mM final concentration), the classical inhibitor of organic anion transport process. The magnitude of pigment uptake from the perfusion medium, estimated by a first order exponential decay constant, was decreased in a dose-dependent way by NA (40 and 76% decrease for 0.1 and 1.0 mM of NA, respectively) and Prob (57 and 88% decrease for 0.1 and 1.0 mM of Prob, respectively). NA and Prob also induced a diminution in the ratio of pigment renal clearance to glomerular filtration rate (24 and 48% decrease for 0.1 and 1.0 mM of NA and 52 and 55% decrease for 0.1 and 1.0 mM of Prob). Based on these findings, it can be proposed that tubular secretion through the proximal cells contributes significantly to renal pigment depuration. In order to establish the possible contribution of cellular metabolism to the secretory process, a different set of experiments was conducted. The content of bilirubin mono and diconjugates (BMC and BDC) were determined in urine, in arterial and venous samples and in renal cortex. Studies performed using either an open or a closed circulating system, revealed that after conjugation in the renal cell, pigment derivatives can be secreted into both the tubule and the venous compartments. Total bilirubin concentration as well as the relative content of BMC and BDC in urine increased over time, representing the sum of both conjugates about 50% of the total pigment excreted by the end of experiments. Consequently, our results support the existence of a tubular transepithelial transport of bilirubin, playing the metabolism of the pigment an important role in this process.

Oxidation of bilirubin by brain mitochondrial membranes--dependence on cell type and postnatal age

Bilirubin is oxidized by brain mitochondrial membranes at a rate which may contribute significantly to clearance of bilirubin from the brain. Neurons appear to be more sensitive to bilirubin toxicity than glial cells. Clinical experience has suggested that sensitivity to bilirubin neurotoxicity may be greater in the neonate than later in life. We hypothesized that the ability to oxidize bilirubin would be lower in mitochondrial membranes from a pure neuronal compared to a mixed glial/neuronal source, and lower in immature than more mature brains. Mitochondria of synaptosomal and nonsynaptosomal origin were obtained from young rat brains by differential centrifugation in sucrose gradients. Synaptosomes were lysed by hypoosmotic treatment, and mitochondria were ruptured by sonication. The change in optical density of a bilirubin solution at 440 nm was measured over time following addition of the membrane suspension. The rate of bilirubin oxidation was significantly higher in nonsynaptic than in synaptic mitochondrial membranes [99.1 +/- 42.3 (mean +/- SD) vs 69.9 +/- 30.9 pmol/ min/mg protein, t = 4.835, P = 0.0003]. "Crude" mitochondrial membranes were obtained by differential centrifugation in sucrose from the forebrains of rats of 7, 14, and 21 days postnatal age as well as adults, and from rabbits of 1 and 7 days postnatal age as well as adults. In both species the rates of bilirubin oxidation increased significantly with postnatal age (rats: F = 55.3, P < 0.0001; rabbits: F = 101, P < 0.0001). Mitochondrial membranes from a pure neuronal source oxidize bilirubin at a significantly lower rate than membranes from a mixed glial/neuronal source. This suggests that neurons may be less able to detoxify bilirubin locally and thus might contribute to the apparent higher sensitivity to bilirubin toxicity in these cells vs glia. Similarly, the lower bilirubin-oxidizing ability of mitochondrial membranes from immature brains seems compatible with the clinical impression of increased vulnerability to bilirubin neurotoxicity in the newborn.