Role of intestinal microflora in metabolism of glutathione conjugates of 1-nitropyrene 4,5-oxide and 1-nitropyrene 9,10-oxide

Xenobiotics: Interaction with the Intestinal Microflora

The human body is host to 100 trillion gut microbes, approximately 10-times more than all human cells. It is estimated that the approximately 500-1000 species residing in the human gut encode 150-fold more unique genes than the human genome. The gut microbiota has important functions in metabolic processing, such as energy production, immune cell development, food digestion, and epithelial homeostasis. It has been increasingly recognized that a dysregulated gut microbiome contributes in a significant way to a variety of diseases, including diabetes, obesity, cardiovascular diseases, allergies, and inflammatory bowel disease. In particular, accumulating evidence indicates that functional interactions between the gut microbiome and xenobiotics play a role in mediating chemical toxicity and causing or exacerbating human disease. This review summarizes emerging evidence that illustrates how xenobiotics can affect the gut microbiome structure, create functional changes to the gut microbiome, and become biotransformed by the gut microbiome.

Role of intestinal microflora in xenobiotic-induced toxicity

In addition to its role in digestion of food in the gastrointestinal tract, the intestinal microflora is also capable of biotransforming numerous drugs. Likewise, the intestinal microflora may significantly modulate xenobiotic-induced toxicity by either activating or inactivating xenobiotics via metabolism. To date, most investigations of xenobiotic metabolism have focused not only on metabolism in host tissues, but the modulation of the pharmacological activity of drugs by the intestinal microflora. Despite its importance, the presumed role of intestinal microflora metabolism in xenobiotic-induced toxicity has been understudied. Therefore, it is appropriate to briefly review our current situation, and state which research in xenobiotic metabolism by intestinal microflora, particularly in the field of toxicology, is needed.

Biological fate of [14C]-1-nitropyrene in rats following intragastric administration

1-Nitropyrene (1-NP), a weak carcinogen associated with diesel exhaust particles, has previously been detected in workplace atmospheres with in-use diesel engines and in the general environment. In order to gain insight in its biological fate, a single dose of [14C]-1-NP (27.6 microCi, 750 mg/kg body weight, b.w.) was administered intragastrically to rats and the presence of metabolites in blood and tissue homogenates, and radioactivity associated with blood proteins and tissue DNA, were studied. Early peak levels of radioactivity observed in blood and tissue homogenates indicated a rapid absorption of [14C]-1-NP from the gastrointestinal tract. Metabolite patterns observed in plasma, liver and kidney homogenates strongly suggested an important role of the intestinal microflora in the enterohepatic recirculation, but not in nitroreduction of 1-NP prior to absorption from the gastrointestinal tract. This might explain the low levels of radioactivity associated with blood proteins, since 1-nitrosopyrene, a product of nitroreduction of 1-NP, is likely to be involved in protein binding. Levels of radioactivity associated with plasma proteins were approximately four times higher than the levels of radioactivity associated with hemoglobin (401.0 and 84.1 pmol/g protein per micromol 1-NP kg b.w., respectively, at 24 h). Maximal 25% of the associated radioactivity was released following mild alkaline hydrolysis of either hemoglobin or plasma proteins. 1-Aminopyrene was the only released compound after hydrolysis of hemoglobin. In addition to 1-aminopyrene, two more polar unidentified metabolites were detected following hydrolysis of plasma proteins. Association of radioactivity with DNA was highest in the liver at the first moments of observation (7.4 pmol 14C Eq./mg DNA per micromol 1-NP kg b.w.), but decreased rapidly to levels lower than observed for kidney DNA (max. 3.0 pmol 14C Eq./mg DNA per micromol 1-NP kg b.w. at 24 h). In lungs 8-50 times less radioactivity was associated with DNA than observed in the liver and kidneys. The results of this study show, that 1-NP undergoes an extensive and complex biotransformation in vivo, resulting in a variety of metabolites present in blood and tissue homogenates and a diversity of blood protein adducts. Concentrations of plasma metabolites, blood protein adducts and DNA adducts were rather low. In addition, previous studies also showed relatively low concentrations of metabolites present in urine. Therefore, sensitive and selective methods will be needed in order to evaluate the biological fate of 1-NP, associated with diesel exhaust particles, in humans.

Determination of hemoglobin adducts following oral administration of 1-nitropyrene to rats using gas chromatography-tandem mass spectrometry

1-Nitropyrene (1-NP) has successfully been used as a marker for environmental monitoring of exposure to diesel exhaust. This study presents a sensitive and selective method for detection of Hb adducts after oral administration of a single dose 1-NP to rats, by measuring 1-aminopyrene (1-AP) after in vitro hydrolysis of the adducts. Released 1-AP was extracted with hexane and derivatized with heptafluorobutyric acid anhydride prior to GC-MS-MS analysis. Optimal conditions for the release of 1-AP were hydrolysis under nitrogen, in 1 M NaOH at 70 degrees C for 60 min. Analysis of a stock solution of Hb adducts of 1-NP utilizing these conditions showed to be reproducible over a period of several weeks with a coefficient of variance of 9.5%. The determination limit was 10-20 pg 1-AP per 70-90 mg globin. A study of the time course of Hb adduct formation showed a fast absorption and an early peak concentration of released 1-AP, approximately 39 pg 1-AP/mg globin at 3 h after exposure. After the maximum was reached, 1-AP concentrations decreased bi-phasically. Initially a fast decline was observed, followed by a slow decrease to 5.9+/-1.9 pg 1-AP/mg globin at 24 h after administration.

The metabolism of drugs by the gut flora

Gut flora and gut contents can be considered as a system with huge metabolic capacity, qualitatively and quantitatively different from the body cells and organs. That system changes along with life and nutrition, but despite broad investigation has not yet been defined satisfactorily. In many cases inter individual and intra individual differences in drug metabolism could be linked to variations in the gut flora metabolism. Gut flora metabolism of drugs and other xenobiotic metabolites excreted in bile is the key phase responsible for enterohepatic circulation. In the last decade there has been more and more evidence for the crucial role of the gut flora cysteine conjugate beta-lyase in the metabolism of cysteine conjugates. A new pathway for paracetamol cysteine conjugate metabolism has been directly linked with gut flora activity, as demonstrated in our studies. Nowadays, it is quite clear that gut flora metabolism must be considered an integral part of drug metabolism and toxicity studies.