Nutritional status modulates rat liver cytochrome P450 arachidonic acid metabolism

Alterations in nutritional status affect hepatic cytochrome P450 levels. Since cytochromes P450 participate in the metabolism of arachidonic acid, we hypothesized that changes in liver P450 arachidonic acid metabolism occur during fasting and refeeding. Male Fisher 344 rats were either fed, fasted 48 hr (F48), fasted 48 hr and then refed 6 hr (F48/R6), or fasted 48 hr and then refed 24 hr (F48/R24). F48 rats had reduced body weight, increased plasma beta-hydroxybutyrate, and reduced plasma insulin compared with the other groups. Although there was no significant change in total liver P450 content, there was a significant 20%, 48%, and 24% reduction in total hepatic microsomal arachidonic acid metabolism in F48, F48/R6, and F48/R24 rats, respectively, compared with fed rats. Epoxygenase activity decreased by 28%, 51%, and 26% in F48, F48/R6, and F48/R24 rats, respectively. In contrast, omega-1 hydroxylase activity increased by 126% in F48 rats compared with fed rats. Immunoblotting revealed that levels of CYP2C11 protein were markedly reduced, whereas levels of CYP2E1 protein were markedly increased in the F48 and F48/R6 groups. In contrast, levels of CYP1A1, CYP1A2, CYP2B1, CYP2J3, CYP4A1, and CYP4A3 were unchanged with fasting/refeeding. Northern blots revealed that levels of CYP2C11 mRNAs were decreased, whereas CYP2E1 mRNAs were increased in F48 and F48/R6 rats. Recombinant CYP2C11 metabolized arachidonic acid primarily to epoxides with preference for the 14(S),15(R)-, 11(R), 12(S)-, and 8(S),9(R)- epoxyeicosatrienoic acid enantiomers. We conclude that (1) nutritional status affects hepatic microsomal arachidonic acid metabolism, (2) reduced epoxygenase activity in F48 and F48/R6 rats is accompanied by decreased levels of CYP2C11, (3) increased omega-1 hydroxylase activity is accompanied by augmented levels of CYP2E1, and (4) the effects of fasting on CYP2C11 and CYP2E1 expression occur at the pretranslational level.     

Hydrogen peroxide supports human and rat cytochrome P450 1A2-catalyzed 2-amino-3-methylimidazo[4,5-f]quinoline bioactivation to mutagenic metabolites: significance of cytochrome P450 peroxygenase

We show that the naturally occurring hydroperoxide hydrogen peroxide is highly effective in supporting the cytochrome P450 1A2 peroxygenase-catalyzed metabolic activation of the heterocyclic aromatic amine 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) to genotoxic metabolites. Mutagenicity was assessed by the Ames assay with Salmonella typhimurium strain YG1012 and an activation system consisting of hydroperoxides plus either 3-methylcholanthrene-induced rat liver microsomes (rP4501A) or human P450 1A2-containing microsomes (hP4501A2). The mutagenic response was dependent on the concentration of microsomal protein, IQ, and hydroperoxides. The addition of hydrogen peroxide or tert-butyl hydroperoxide to rP4501A greatly enhanced the yield of histidine prototrophic (His+) revertants. This increase was inhibited, in a concentration-dependent manner, by alpha-naphthoflavone, a P450 1A inhibitor. Hydrogen peroxide was the most effective peroxygenase cofactor, particularly with hP4501A2 (K(m) = 0.1 mM). The hydroperoxide-supported activation of IQ produced reactive intermediates which bound to 2'-deoxyguanosine; LC/MS analysis of the adducts revealed the same major (protonated) adduct at m/z = 464.4 as previously reported for the DNA adduct formed (in vivo or in vitro) by the mixed function-catalyzed bioactivation system. None of the peroxidase-catalyzed IQ metabolites (nitro-, azo-, or azoxy-IQ) were detected. In conclusion, hydrogen peroxide in the physiological/pathological concentration range may be able to support the metabolic activation of arylamines to genotoxic products through the cytochrome P450 peroxygenase pathway.     

Suppression of male-specific cytochrome P450 isoforms by bisphenol A in rat liver

IBD results from the interaction of genetic and environmental factors (e.g., smoking). Clinical suspicion is the key to diagnosis, which then rests on colonoscopy, histopathological examination of multiple biopsy specimens, small bowel barium radiology and faecal examination. The primary goal of treatment is remission--histological in ulcerative colitis and symptomatic in Crohn's disease. Treating active disease and maintaining remission require different approaches. For active disease, short term corticosteroids are the mainstay of treatment, while immunosuppressive drugs are important in chronically active disease. For maintenance, mesalazine-delivering drugs and immunosuppressive agents are efficacious in both ulcerative colitis and Crohn's disease; patients with Crohn's disease should not smoke.     

A structure-based model for cytochrome P450cam-putidaredoxin interactions

Putidaredoxin (Pdx) is a Fe2S2 ferredoxin which acts as the physiological reductant of cytochrome P-450cam (CYP101). A model for the solution structure of oxidized Pdx has been determined using NMR methods (Pochapsky et al (1994) Biochemistry 33, 6424-6432). 1H-15N correlations and redox-dependent amide exchange rates have also been described (Lyons et al (1996) Protein Sci 5, 627-639). Data obtained from mutagenesis and kinetic measurements concerning the interactions of Pdx and CYP101 are summarized. A model for the structure of the homologous ferredoxin adrenodoxin (Adx) is also described, and data concerning Adx activity are discussed in relation to this structure. The structures of Pdx and CYP101 were used as starting points for molecular modeling and molecular dynamics simulations. Close approach between the metal centers of the two proteins and interaction between aromatic residues on the surfaces of the proteins are premised. The resulting complex exhibits three intermolecular salt bridges, five intermolecular hydrogen bonds and a 12 A distance between the metal centers. The first direct observations of interaction between Pdx and CYP101 (by two-dimensional NMR of 15N-labeled Pdx in solution with CYP101) are described. The results of the NMR experiments indicate that conformational gating of the electron transfer complex between CYP101 and Pdx may be important.     

Isozyme selective alterations of the expression of cytochrome P450 during regeneration of male rat liver following partial hepatectomy

1. During liver regeneration in the male rat, the metabolic activities of imipramine were differentially affected depending on the specific metabolic pathways. Imipramine N-demethylation was markedly reduced whereas 2-hydroxylation showed only a moderate reduction following partial hepatectomy. 2. A slight decline was observed in the hepatic microsomal content of CYP2D apoprotein, whereas a substantial decrease occurred in CYP2C11 content during liver regeneration. Since imipramine 2-hydroxylation and N-demethylation are mediated by CYP2D and 2C11 respectively, metabolic pathway-specific alterations in the activities of imipramine metabolism are explained by the isozyme selective alteration in the levels of CYPs in regenerating liver. 3. No significant effect of regeneration was observed on expression of CYP2B1 and 2E1 apoproteins. CYP3A2 apoprotein, one of the male-specific CYP isoforms, was significantly suppressed in regenerating liver showing a similar pattern of alteration to the levels of CYP2C11. The alteration pattern of the CYP1A1 level was different to the above with a moderate decline at the first day post-operation and a marked rebound thereafter. 4. In the partially hepatectomized male rate, no significant increase in androstenedione 5-alpha reductase activity, an activity predominant in the female rat, was detected. It is concluded that the pattern of alterations of hepatic oxidative metabolism during liver regeneration was not related to the functional feminization of the liver.     

Peroxo-iron and oxenoid-iron species as alternative oxygenating agents in cytochrome P450-catalyzed reactions: switching by threonine-302 to alanine mutagenesis of cytochrome P450 2B4

Among biological catalysts, cytochrome P450 is unmatched in its multiplicity of isoforms, inducers, substrates, and types of chemical reactions catalyzed. In the present study, evidence is given that this versatility extends to the nature of the active oxidant. Although mechanistic evidence from several laboratories points to a hypervalent iron-oxenoid species in P450-catalyzed oxygenation reactions, Akhtar and colleagues [Akhtar, M., Calder, M. R., Corina, D. L. & Wright, J. N. (1982) Biochem. J. 201, 569-580] proposed that in steroid deformylation effected by P450 aromatase an iron-peroxo species is involved. We have shown more recently that purified liver microsomal P450 cytochromes, including phenobarbital-induced P450 2B4, catalyze the analogous deformylation of a series of xenobiotic aldehydes with olefin formation. The investigation presented here on the effect of site-directed mutagenesis of threonine-302 to alanine on the activities of recombinant P450 2B4 with N-terminal amino acids 2-27 deleted [2B4 (delta2-27)] makes use of evidence from other laboratories that the corresponding mutation in bacterial P450s interferes with the activation of dioxygen to the oxenoid species by blocking proton delivery to the active site. The rates of NADPH oxidation, hydrogen peroxide production, and product formation from four substrates, including formaldehyde from benzphetamine N-demethylation, acetophenone from 1-phenylethanol oxidation, cyclohexanol from cyclohexane hydroxylation, and cyclohexene from cyclohexane carboxaldehyde deformylation, were determined with P450s 2B4, 2B4 (delta2-27), and 2B4 (delta2-27) T302A. Replacement of the threonine residue in the truncated cytochrome gave a 1.6- to 2.5-fold increase in peroxide formation in the presence of a substrate, but resulted in decreased product formation from benzphetamine (9-fold), cyclohexane (4-fold), and 1-phenylethanol (2-fold). In sharp contrast, the deformylation of cyclohexane carboxaldehyde by the T302A mutant was increased about 10-fold. On the basis of these findings and our previous evidence that aldehyde deformylation is supported by added H202, but not by artificial oxidants, we conclude that the iron-peroxy species is the direct oxygen donor. It remains to be established which of the many other oxidative reactions involving P450 utilize this species and the extent to which peroxo-iron and oxenoid-iron function as alternative oxygenating agents with the numerous isoforms of this versatile catalyst.     

Inhibition of human cytochrome P450-catalyzed oxidations of xenobiotics and procarcinogens by synthetic organoselenium compounds

The effects of synthetic chemopreventive organoselenium compounds 1,2-, 1,3-, and 1,4-phenylenebis(methylene)selenocyanate (o-, m-, and p-XSC, respectively), benzyl selenocyanate (BSC), and dibenzyl diselenide (DDS) and inorganic sodium selenite on the oxidation of xenobiotics and procarcinogens by human cytochrome P450 (P450 or CYP) enzymes were determined in vitro. Spectral studies showed that BSC and three XSC compounds (but not sodium selenite or DDS) induced type II difference spectrum when added to the suspension of liver microsomes isolated from beta-naphthoflavone-treated rats, with m-XSC being the most potent in inducing spectral interactions with P450 enzymes; m-XSC also produced a type II spectral change with human liver microsomes. o-, m-, and p-XSC inhibited 7-ethoxyresorufin O-deethylation catalyzed by human liver microsomes when added at concentrations below 1 microM levels, but BSC and DDS were less effective. All of these compounds inhibited the oxidation of model substrates for human P450s to varying extents. We studied the effects of these compounds on the activation of procarcinogens by recombinant human CYP1A1, 1A2, and 1B1 enzymes using Salmonella typhimurium NM2009 tester strain for the detection of DNA damage. The three XSCs were found to be very potent inhibitors of metabolic activation of 3-amino-1,4-dimethyl-5H-pyrido[4,3-b]indole, 2-amino-3,5-dimethylimidazo[4,5-f]quinoline, and 2-aminoanthracene, catalyzed by CYP1A1, 1A2, and 1B1, respectively. The potency of inhibition of m-XSC on CYP1B1-dependent activation of 2-aminoanthracene was compatible to those of alpha-naphthoflavone. These inhibitory actions may, in part, account for the mechanisms responsible for cancer prevention by organoselenium compounds in laboratory animals.     

Influence of substituents in fluorobenzene derivatives on the cytochrome P450-catalyzed hydroxylation at the adjacent ortho aromatic carbon center

In the present study, the effects of extracellular magnesium concentration ([Mg2+]ex) on stimulus-secretion coupling processes were investigated in rat gastric parietal cells in vitro. Extracellular magnesium reduction resulted in (1) an increase of basal intracellular free calcium concentration ([Ca2+]in), (2) an enhancement of both carbachol and thapsigargin-induced calcium responses, (3) an improved filling state of intracellular calcium stores, (4) an increase of both basal and carbachol-induced acid secretion, whereas intracellular adenosine 3',5'-cyclic monophosphate (cyclicAMP) levels and histamine stimulated acid secretion were not affected. The effects of high [Mg2+]ex were opposite to the described results, except that high [Mg2+]ex was able to decrease significantly histamine-stimulated cyclicAMP levels and acid secretion. These findings indicate a modulatory role of [Mg2+]ex on the intracellular signalling processes and acid secretory properties in rat parietal cells. These effects seemed to be mediated by regulating (1) calcium loading capacity of intracellular stores, (2) the permeability of the calcium influx pathway, and (3) the formation of cyclicAMP.     

Antiprogestin-mediated inactivation of cytochrome P450 3A4

Interleukin-12 (IL-12) is a heterodimeric cytokine that is central to the development of T helper 1-dependent cellular immunity. Although this cytokine has potential therapeutic application as an antineoplastic agent, the systemic infusion of IL-12 has led to toxic fatalities; hence, restriction of expression of IL-12 to the microenvironment of target tumor cells has obvious appeal. In this study, we examined whether tumor cells that were liposome-transfected with IL-12 could enhance the induction of cytolytic lymphocyte immunity to the native tumor. The plasmid expression vector that we used has several useful features including replication to high copy number as an episome and a polycistronic message enabling the production of both the p35 and p40 subunits of IL-12 without alternative splicing; up to 3 ng/mL/10(6)/48 hours of IL-12 was produced following transfection. Tumor cells transfected with IL-12 were superior to untransfected cells in the induction of lymphocyte-mediated cytolysis. IL-12 transfectants induced a heterogeneous population of natural killer, lymphokine activated killer, and cytolytic T lymphocytes, the latter of which exhibited tumor-specific activity. Our studies suggest that liposome-mediated transfection of tumor cells with an episomal, high copy number plasmid vector expressing both IL-12 subunits is a promising approach to cancer vaccination, a strategy that could be implemented ex vivo in treating malignancies such as metastatic ovarian cancer.     

Escherichia coli expression and substrate specificities of canine cytochrome P450 3A12 and rabbit cytochrome P450 3A6

High level Escherichia coli expression of cytochromes P450 3A12 and 3A6 has facilitated the characterization of proteins which exhibit limited activity as purified hepatic enzymes in reconstituted systems. Three 3A12 and two 3A6 constructs modified at the 5'-end to encode the bovine 17 alpha-sequence (Barnes et al., Proc. Natl. Acad. Sci. U.S.A. 88: 5597-5601, 1991), or related sequences, exhibited expression levels ranging from 2 to 89 nmol of cytochrome P450 liter-1. Recombinant canine 3A12 catalyzed steroid 6 beta-hydroxylation and erythromycin demethylation at rates comparable to those obtained in phenobarbital-induced canine liver microsomes. In contrast, 3A12 troleandomycin demethylase activity (2.5 nmol/min/nmol) was significantly lower than that of canine phenobarbital-induced liver microsomes (6.6 nmol/min/nmol). This difference in activity suggests that at least two 3A forms, which may differ functionally, are present within the canine liver. Purification of recombinant rabbit 3A6 revealed that homogeneous and E. coli-solubilized membrane preparations of 3A6 exhibit similar metabolic rates and identical substrate specificities; 3A activity was modulated by 25 microM alpha-naphthoflavone, which stimulated an unidentified progesterone metabolite 9-fold in 3A6 reconstituted systems in contrast to the 4-fold stimulation of 3A12. Furthermore, 25 microM alpha-naphthoflavone inhibited erythromycin demethylation 64 and 33% by purified recombinant 3A6- or 3A6-solubilized membrane fractions, respectively; 3A12-mediated erythromycin demethylation in solubilized membrane fractions was resistant to flavonoid inhibition. These results indicate that, although 3A substrate specificities are highly conserved between species, functional differences exist between canine 3A12 and rabbit 3A6, which may be utilized to better understand 3A structure-function relationships.     

Positive feedback between ethanolamine-specific phospholipid base exchange and cytochrome P450 activities in rat liver microsomes. The effect of clofibric acid

The results of the present investigation relate the effects of the nutritional state and administration of clofibric acid (CLA), a hypolipidaemic drug and peroxisomal proliferator, on phosphatidylethanolamine (PE) synthesis in rat liver and fatty acid metabolism. Fasting and CLA treatment of animals causes an increase in the amount of PE in endoplasmic reticulum (ER) membranes and mitochondria, as well as in the PE/phosphatidylcholine (PC) ratio. Moreover, the activity of the ethanolamine-specific phospholipid base exchange (PLBE) enzyme in liver ER membranes of fasted animals was enhanced by 75% in comparison to that of animals fed ad libitum. The effect of CLA treatment was additive to that of starvation; PE synthesis tested in vitro via the Ca2+-sensitive PLBE reaction increased 3-fold in comparison to rats fed ad libitum. This is confirmed by an increased Vmax for the reaction, but the affinity of the enzyme for ethanolamine was not significantly changed. These effects were accompanied by an enhanced expression of cytochrome P450 CYP4A1 isoform and elevated activity of the enzyme upon CLA administration. The stimulatory effect of CLA administration on the efficiency of the ethanolamine-specific PLBE reaction can be explained by elimination of lauric acid, a known inhibitor of de novo PE synthesis, during the course of omega-hydroxylation catalysed by CYP4A1, and by increased expression of the PLBE enzyme. The products of omega-hydroxylation of lauric acid, which are then converted by dehydrogenase to 1,12-dodecanedioic acid, did not significantly affect the in vitro synthesis of PE.     

Significance of hepatic cytochrome P450 enzymes for psychopharmacology

Nearly all psychotropic drugs are metabolized by hepatic cytochrome P450-enzymes. In humans, there are 5 isoenzymes involved in this process. The activity of these enzymes can be modulated by a number of commonly used drugs, yielding potentially hazardous interactions. Most of the recently introduced selective serotonin reuptake inhibitors are potent inhibitors of cytochrome P450 enzymes. Thus, the plasma concentrations of tricyclic antidepressants or clozapine might be elevated into toxic levels. In contrast, carbamazepine induces most of the isoenzymes. This potentiates the elimination of tricyclics and antipsychotics and might cause a serious risk for the recurrence of depressive or psychotic symptoms. Moreover, 5-10% of the population are slow metabolizers of CYP2D6. This group is prone to increased adverse effects of moderately dosed medication. This review systematically points out the reported or predicted pharmacokinetic drug interactions in psychopharmacology focussing on clinical significance.     

Synergistic increases in rat hepatic cytochrome P450s by ethanol and isopentanol

The purpose of this study was to determine if isopentanol alone or in combination with ethanol increased CYP2B1/2, CYP2E or CYP3A in the livers of rats. Increasing doses of isopentanol (0.5, 1, 2 or 3%) were administered in combination with 5.6% ethanol in the Lieber-DeCarli liquid diet for 7 days. Doses of 0.5 or 3% isopentanol were also administered alone. Isopentanol alone caused small increases in CYP2B1/2 and CYP3A. However, when isopentanol (2 or 3%) was combined with ethanol a synergistic increase in P4502B1/2 was observed. The combined alcohol treatment also resulted in a greater increase in immunoreactive CYP3A than either alcohol alone. Ethanol alone increased CYP2E 5-fold. Inclusion of isopentanol with ethanol resulted in either small or no additional increases in CYP2E. These results confirm our previous findings in cultured hepatocytes that when isopentanol is combined with ethanol, there is a synergistic increase in CYP2B1/2. Increases in CYP2B1/2, CYP2E and CYP3A protein moieties by ethanol, and by ethanol in combination with isopentanol, were associated with increases in their mRNAs. Blood isopentanol levels were 10-fold greater in rats administered 3% isopentanol in combination with ethanol compared to rats administered 3% isopentanol alone. From these results we suggest that isopentanol, a higher chain alcohol in alcoholic beverages, can contribute to increases in hepatic cytochrome P450 observed following consumption of alcoholic beverages.     

The emerging role of cytochrome P450 3A in psychopharmacology

Recent advances in molecular pharmacology have allowed the characterization of the specific isoforms that mediate the metabolism of various medications. This information can be integrated with older clinical observations to begin to develop specific mechanistic and predictive models of psychotropic drug interactions. The polymorphic cytochrome P450 2D6 has gained much attention, because competition for this isoform is responsible for serotonin reuptake inhibitor-induced increases in tricyclic antidepressant concentrations in plasma. However, the cytochrome P450 3A subfamily and the 3A3 and 3A4 isoforms (CYP3A3/4) in particular are becoming increasingly important in psychopharmacology as a result of their central involvement in the metabolism of a wide range of steroids and medications, including antidepressants, benzodiazepines, calcium channel blockers, and carbamazepine. The inhibition of CYP3A3/4 by medications such as certain newer antidepressants, calcium channel blockers, and antibiotics can increase the concentrations of CYP3A3/4 substrates, yielding toxicity. The induction of CYP3A3/4 by medications such as carbamazepine can decrease the concentrations of CYP3A3/4 substrates, yielding inefficiency. Thus, knowledge of the substrates, inhibitors, and inducers of CYP3A3/ and other cytochrome P450 isoforms may help clinicians to anticipate and avoid pharmacokinetic drug interactions and improve rational prescribing practices.     

Changes in rat liver cytochrome P450 enzymes by atrazine and simazine treatment

1. We examined the effect of two chloro-s-triazines (atrazine and simazine) on hepatic microsomal cytochrome P450 enzymes in rat. Rats were treated intraperitoneally with atrazine or simazine daily for 3 days with 100, 200 and 400 mumol/kg. 2. Among the P450-dependent monooxygenase activities, testosterone 2 alpha-hydroxylase (T2AH) activity in rat, which is associated with CYP2C11, was significantly decreased at all doses of atrazine and simazine. The levels relative to control activities were 59-46 and 60-32% respectively. Similarly, oestradiol 2-hydroxylase (ED2H) activity was also significantly decreased by 28-51% by atrazine and simazine at all doses. However, no change in CYP2C11 protein level by either chloro-s-triazine was observed. K(m) for T2AH was significantly increased only by simazine (200 mumol/kg), whereas the Vmax and Cl(int) for T2AH were significantly decreased by atrazine and simazine at all doses. 3. 7-Ethoxyresorufin O-deethylase (EROD), 7-methoxyresorufin O-demethylase (MROD) and 7-pentoxyresorufin O-depentylase (PROD) activities were significantly increased by 1.4-1.6-, 1.7-3.2- and 1.5-2.2-fold respectively, by both chloro-s-triazines at 200 or 400 mumol/kg. Lauric acid omega-hydroxylase (LAOH) was also increased by 1.4-fold by simazine at 200 and 400 mumol/kg. Immunoblotting showed that only simazine induces CYP1A2 and CYP4A1/2 protein expression. 4. The activities of 7-ethoxycoumarin O-deethylase (ECOD), bufuralol 1'-hydroxylase (BF1'H), chlorzoxazone 6-hydroxylase (CZ6H), testosterone 6 beta-hydroxylase (T6BH) and testosterone 7 alpha-hydroxylase (T7AH) were not affected by either chloro-s-triazine. 5. These results suggest that the pattern of changes in P450 isoforms by chloro-s-triazines differs between atrazine and simazine, that these herbicides change the constitutive and/or male specific P450 isoform(s) in rat liver, and that these changes closely relate to the toxicity of chloro-s-triazines.     

Dose-response relationships for cytochrome P450 induction by phenobarbital in the cotton rat (Sigmodon hispidus)

There is immunohistochemical evidence suggesting that glutamate (Glu) is released from nerve terminals and acts, via several receptor subtypes, as a major excitatory neurotransmitter in the cortico-striatal pathway of the rat. Aspartate (Asp) is also present in cortico-striatal neurons, but its role as a neurotransmitter has been questioned, since, in contrast to Glu, it has not been demonstrated in presynaptic vesicles. Glu and Asp can be found at submicroM concentrations in the extracellular compartment of most areas of the basal ganglia. Their concentrations are largely regulated by transport mechanisms, but also by a synaptotagmin-dependent exocytotic release, and are sufficiently high to occupy junctional and extrajunctional receptors. We have investigated whether Glu and Asp release in the neostriatum can be selectively modulated by different neuronal systems. Dopamine (DA) and cholecystokinin (CCK) selectively stimulate Asp release, via D1 and CCKB receptor subtypes, respectively. Also opioid kappa-agonists increase Asp release. We propose that the selective modulation of Asp release by D1-, CCKB- and kappa-agonists involves striatal neurons containing Asp, but not Glu. In contrast, local perfusion with the mu-opioid antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP) increases both Glu and Asp release. This effect is probably exerted on cortico-striatal terminals, via presynaptic inhibitory mu-receptors. Thus, these results demonstrate that extracellular levels of Glu and Asp are modulated differentially by different neuronal systems, and suggest that in the neostriatum of the rat there are neuronal populations using Glu and/or Asp as messenger(s).     

A refined substrate model for human cytochrome P450 2D6

Human epidermal keratinocytes possess cholinergic enzymes, which synthesize and degrade acetylcholine, and express both nicotinic and muscarinic classes of cholinergic receptors on their cell surfaces. These receptors bind acetylcholine and initiate cellular response. The presence in keratinocytes of a functional cholinergic system suggests a role for acetylcholine in most, if not all, aspects of keratinocyte function. Autocrine and paracrine acetylcholine are required to sustain the viability of keratinocytes in vitro, and cholinergic drugs can alter keratinocyte proliferation, adhesion, migration, and differentiation. Acetylcholine employs calcium as a mediator for its effects on keratinocytes. In turn, changes in calcium concentration may affect expression and function of keratinocyte cholinergic enzymes and cholinergic receptors. At different stages of their differentiation, keratinocytes may demonstrate unique combinations of cholinergic enzymes and cholinergic receptor types. This would allow basal, prickle, and granular keratinocytes to respond to acetylcholine differently, in accordance with their functions at each stage of keratinocyte development in epidermis.