Page 283 - Veterinary Toxicology, Basic and Clinical Principles, 3rd Edition

250 SECTION | II Organ Toxicity

VetBooks.ir unpredictable responses to a drug or other xenobiotic. They N CHO CH 3 N CO CH 3
contrast,
by
Idiosyncratic
reactions
are,
are rare, not dependent on dose, and can be associated with
extrahepatic lesions (Sturgill and Lambert, 1997; CYP450
Zimmerman, 1999; Pineiro-Carrero and Pineiro, 2004; OH O
NAPQI
Shenton et al., 2004). Extrahepatic clinical signs can
GSH
include pyrexia, rash, and peripheral eosinophilia. Some
idiosyncratic drug reactions resemble serum sickness. Glucuronidation, Mercapturic Necrosis
sulfation
Hepatic lesions associated with idiosyncratic drug reactions acid (covalent binding)
include necrosis, cholestasis, or both, and there is often an
inflammatory response involving macrophages and FIGURE 15.4 Metabolism and mechanism of acetaminophen toxicity.
Bioactivation of acetaminophen by P450 enzymes results in the forma-
eosinophils.
tion of the reactive intermediate (NAPQI) that forms covalent adducts
with glutathione which is then converted to mercapturic acid. When the
Oxidative Stress Mediated by Free Radicals amount of the reactive metabolite formed exceeds the glutathione avail-
able for binding, the excess metabolite binds to tissue molecules, form-
ing covalent adducts, resulting in centrilobular hepatic necrosis.
Free radicals are generated from within hepatocytes
through a variety of mechanisms: oxidative metabolism
by cytochrome P450s, reduction and oxidation (redox) produced by these compounds are usually centrilobular due
reactions of normal metabolism, transition metals such as to the location of the cytochrome P450s responsible.
iron and copper, nitric oxide generated by inflammatory A classic example of phase I bioactivation is cell
cells, and ion radiation. Free radicals produce lipid perox- death resulting from acetaminophen (Fig. 15.4).
idation of membranes, oxidative modification of proteins, Acetaminophen is a widely used analgesic that rarely
and DNA disruption (Crawford, 1999). induces clinical signs at therapeutic doses, except in the
Free radicals have unpaired electrons, making them particularly susceptible feline species where metabolites
highly reactive. They are formed by one electron oxidation of acetaminophen produce acute liver failure and oxida-
and reduction reactions, which produce cationic and tion of hemoglobin to methemoglobinemia. The details
anionic radicals, respectively (Dahm and Jones, 1996). of acetaminophen toxicosis are described in another
Alternately, homolytic bond scission produces neutral chapter, but one unstable intermediate metabolite,
radicals. The free radical nitric oxide ( NO), an important N-acetyl-p-benzoquinone imine (NAPQI), is the primary
cell signaling agent released by leukocytes, reacts with cause of hepatocyte injury.
superoxide to form peroxinitrite. Hydroxyl radicals, super- The reaction that produces NAPQI generates superox-
oxide radicals, and hydrogen peroxide are major ROS. ide anions as a by-product. NAPQI itself also acts as an
Free radicals produce peroxidation of phospholipids electrophile, targeting mitochondria in particular, forming
within the cell plasma membrane, mitochondrial mem- covalent adducts with protein thiol groups and other cellu-
brane, and the endoplasmic reticulum. The radicals lar macromolecules. Interaction of NAPQI with other
1
remove a proton (H ) from a methylene carbon within a cellular molecules generates more ROS, leading to oxida-
polyunsaturated fatty acid, forming a lipid-free radical. This tive stress (Dahm and Jones, 1996; Zimmerman, 1999).
step is called initiation. This lipid-free radical then abstracts Adenine nucleotides and plasma membrane proteins
a proton from a neighboring polyunsaturated fatty acid, involved in calcium homeostasis are also targeted (Dahm
generating more lipid-free radicals. Propagation by this and Jones, 1996; Sturgill and Lambert, 1997). The role of
mechanism is estimated to occur 4 10 times per initiation. Kupffer cell activation has been implicated as contributing
Effects of lipid peroxy radicals on the cell membrane to acetaminophen-induced liver injury through the produc-
include: increased permeability, decreased fluidity, inactiva- tion of reactive nitrogen species (Treinen-Moslen, 2001).
tion of membrane proteins (Dahm and Jones, 1996)and,in Another classic example of xenobiotic bioactivation,
the case of mitochondrial membranes, loss of polarity generation of free radicals, and peroxidative damage is
(Watkins, 1999). Lipid peroxy radicals react with stored conversion of CCl 4 into trichlorocarbon radical ( CCl 3 )
metal ions in hepatocytes to generate more lipid radicals. and then to trichloroperoxy radical (CCl 3 OO )(Fig. 15.5).
The most frequent initiating factor in free radical pro- The main phase I enzyme involved in bioactivation
duction inducing hepatocellular injury is the bioactivation of both aforementioned acetaminophen and CCl 4 is
of xenobiotics by the cytochrome P450 system. Phase I CYP2E1, which can be induced by ethanol consumption
metabolism activates substrates to reactive intermediate in humans. Lipid peroxidation caused by free radicals
21
molecules in preparation for phase II conjugation reactions. increases the Ca permeability of the plasma membrane,
However, in circumstances of high exposure, phase I leading to disruption of calcium homeostasis and centri-
products accumulate (Gu and Manautou, 2012). Lesions lobular cell necrosis. In addition, CCl 3 directly binds to