Distribution and induction of cytochrome P450 1A1 and 1A2 in rat brain.

Cytochromes P450 1A1 and 1A2 are involved in the oxidation of a wide spectrum of endogenous compounds and xenobiotics. Although their presence has been repeatedly confirmed in brain tissue, reports regarding their distribution in the brain are often contradictory. In the present study the possibility was examined that CYP1A1 and CYP1A2 are localized and inducible in the brain-CSF barrier and regions with a leaky blood brain barrier, where they may serve as a protective metabolic barrier. CYP1A1 and CYP1A2 levels were determined in subcellular fractions of multiple brain regions, as well as tissue homogenates of circumventricular organs, and the meninges by Western blotting and catalytic activity in control male rats and rats treated with the inducer beta-naphthoflavone (BNF). In control animals CYP1A1 immunoreactive protein was undetectable in regional brain microsomes or whole tissue homogenates of the arachnoid, dura mater, choroid plexus, pineal gland, median eminence, and pituitary. However, low levels of ethoxyresorufin O-deethylase (EROD) activity were observed in homogenates of the arachnoid, dura mater, choroid plexus, pineal gland, and pituitary. Western blotting revealed only low levels of CYP1A2 immunoreactive protein in brain microsomes from the cortex, cerebellum, brainstem, thalamus, hippocampus, and striatum from control animals. Following BNF treatment, EROD activity was induced 12-42-fold in the arachnoid, choroid plexus, dura mater, pineal gland, pituitary, and median eminence. Western blot analysis revealed CYP1A1 to be induced in the arachnoid, dura mater, choroid plexus, pineal gland, and pituitary, while CYP1A2 was undetectable. No induction of CYP1A1 or CYP1A2 protein was observed in brain microsomes from the olfactory bulb, cortex, striatum, hippocampus, cerebellum, or brainstem following BNF treatment, providing that the arachnoid membranes and choroid plexus had been carefully removed prior to brain dissection. Neither CYP1A1, 1A2 protein, nor EROD activity were detected in purified brain mitochondria, regardless of treatment or region. In conclusion, catalytically active CYP1A1 is located in the meninges as well as certain circumventricular organs, is inducible by BNF, and appears to be absent or expressed constitutively at very low levels in the majority of the brain parenchyma. The localization of CYP1A1 in the blood-CSF barrier and circumventricular tissues likely plays a role in protecting the brain from xenobiotics.

Effects of phenethyl isothiocyanate on acetaminophen metabolism and hepatotoxicity in mice.

Phenethyl isothiocyanate (PEITC), a compound derived from cruciferous and other vegetables, is a potent inhibitor of cytochrome P450 2E1. This enzyme catalyzes the bioactivation of acetaminophen (APAP) and many other xenobiotics. The present study investigated the effects of PEITC on APAP metabolism and associated hepatotoxicity in Swiss-Webster mice. When PEITC (19-150 micromol/kg) was given to mice intragastrically 1 hr before or immediately prior to a toxic dose of APAP, the APAP-induced hepatotoxicity was significantly decreased or was completely prevented. The extent of toxicity was evaluated by mortality, serum levels of glutamic-pyruvic transaminase, lactate dehydrogenase, and liver histopathology. Pretreatment of mice with ethanol enhanced APAP hepatotoxicity; this enhanced toxicity could also be prevented by the administration of PEITC. PEITC treatment prevented the depletion of hepatic glutathione levels caused by oxidized APAP metabolites. PEITC treatment also significantly decreased the plasma levels of oxidized APAP metabolites (analyzed as APAP-glutathione, APAP-cysteine, and APAP-N-acetylcysteine) and reduced the urinary excretion of APAP-cysteine. In microsomal incubations, PEITC effectively inhibited the rate of APAP-glutathione formation from APAP as well as the P450 2E1-dependent N-nitrosodimethylamine demethylase and the P450 1A2-dependent ethoxyresorufin O-deethylase activities. The protective action of PEITC against APAP toxicity is attributed to the blocking of APAP activation through inhibition of P450 enzymes.

Protective effects of garlic and related organosulfur compounds on acetaminophen-induced hepatotoxicity in mice.

In previous studies, we have demonstrated that diallyl sulfide, a flavor component of garlic, protects against chemically induced hepatotoxicity. The present study examined the activities of fresh garlic homogenates (FGH) and related organosulfur compounds in the protection against acetaminophen (APAP)-induced hepatotoxicity and the possible mechanisms involved in this protection. When FGH (5 g/kg) was administered to Swiss-Webster mice 2 hr prior to, or immediately after, an APAP treatment (0.2 g/kg), APAP-induced hepatotoxicity was essentially prevented as indicated by serum levels of alanine aminotransferase and lactate dehydrogenase and by liver histopathology. Partial protection was observed with a lower dose of FGH (0.5 g/kg). FGH also prevented APAP-induced hepatic glutathione depletion in a dose-dependent manner. FGH significantly inhibited the formation of APAP-oxidized metabolites, as indicated by decreased plasma levels of oxidized APAP metabolites. The amount of APAP excreted as oxidized metabolites in the 24 hr urine samples was also significantly lower in the mice pretreated with FGH. FGH supernatant inhibited cytochrome P450-dependent APAP oxidation in microsomal incubations. The results suggest that the protection against APAP-induced hepatotoxicity by FGH is mainly due to its inhibition of P450-mediated APAP bioactivation. Several garlic-derived organosulfur compounds and structurally related compounds were examined for their abilities to protect against APAP-induced hepatotoxicity. An S-allyl structure appears to be a common feature for most sulfides to inhibit P450 2E1-dependent activity and to display good protective activities.

N-cadherin in normal and abnormal brain development.

The brain relies upon numerous morphoregulatory molecules to control cell-cell interactions, cell migration and neurite extension. N-cadherin, a calcium-dependent cell adhesion molecule, is essential for normal CNS development. Homophilic binding of N-cadherin depends upon a specific conformation assumed by the molecule when it binds calcium. N-cadherin is a substrate for a specific zinc-dependent protease that may be involved in the regulation of N-cadherin at the cell surface. The reliance of N-cadherin on two cations for proper function makes it a potential target for toxicants which act by replacing or modifying calcium or zinc at ion-binding sites. Exposure of the developing brain to lead, an ubiquitous toxicant known to interact with calcium, disturbs neural tube closure and subsequent maturation of the nervous system. Preliminary data indicates that lead may induce these effects by direct interaction with N-cadherin. Numerous common toxicants, including metals and solvents, also perturb cadherins and cause defective CNS development. These data indicate that changes in the spatio-temporal expression of cadherin can result in profound alterations in neural structure and function, and may underlie CNS malformations caused by numerous toxic agents.