Drug oxidation and N-acetylation in rats pretreated with subtoxic doses of streptolysin O

Arylamine N-acetyltransferases and drug response

Arylamine N-acetyltransferases (NATs) play an important role in the interaction of competing metabolic pathways determining the fate of and response to xenobiotics as therapeutic drugs, occupational chemicals and carcinogenic substances. Individual susceptibility for drug response and possible adverse drug reactions are modulated by the genetic predisposition (manifested for example, by polymorphisms) and the phenotype of these enzymes. For all drugs metabolized by NATs, the impact of different in vivo enzyme activities is reviewed with regard to therapeutic use, prevention of side effects and possible indications for risk assessment by phenotyping and/or genotyping. As genes of NATs are susceptibility genes for multifactorial adverse effects and xenobiotic-related diseases, risk prediction can only be made possible by taking the complexity of events into consideration.

Influence of pretreatment with immunosuppressants on O-demethylation of dextromethorphan in isolated perfused rat liver

Changes in the immune system induced by exogenous or endogenous factors may be accompanied with modifications of the activity of the drug metabolising enzymes in the liver. Some immunostimulatory agents are known to suppress the oxidative metabolism mediated by cytochromes P450. Possible effects of substances which suppress the immune responses of the organism have not been fully understood yet. The present study was undertaken to investigate the influence of immunosuppressants cyclophosphamide and dexamethasone on the CYP2D 1-dependent metabolism of dextromethorphan (DEM) in the isolated perfused liver from male rat donors (Wistar albino, 250-310 g). Recirculatory perfusion system was used with Williams' medium E (Sigma Chemicals Co.) as a perfusion medium (120 mL). DEM was administered as a 1 mg bolus into the perfusion solution at the start of each experiment after 20 min preperfusion. Samples of perfusate for HPLC determination of DEM and its O-demethylated metabolite dextrorphan (DOR) were taken at 15 min intervals for 120 min. The results have shown a rapid conversion of DEM to DOR in the isolated rat liver preparations. Pharmacokinetic parameters in the livers from intact rats were as follows: t1/2 DEM = 19.1+/-4.10 min, k(m) = 0.035+/-0.008 min(-1), Cl(m) = 4.21+/-0.97 mL x min(-1), AUC(DOR) = 2160+/-201 microg x min x L(-1). Pretreatment of rats with dexamethasone (4 mg/kg/day, iv, x 3 days) led to a significant increase in the concentration of dextrorphan in the recirculating solution, but it did not substantially change the kinetic constants of DOR formation (km = 0.036+/-0.004 min(-1), Cl(m) = 4.27+/-0.43 mL x min(-1)). Cyclophosphamide (100 mg/kg, ip, 1 dose on day 5 before perfusion) induced nearly twofold increase in the DOR concentrations in perfusate and thus highly significant (p < 0.01) changes of the kinetic parameters characterizing the increased rate of conversion of DEM to DOR (t1/2 DEM = 12.1+/-0.90 min, km = 0.055+/-0.004 min(-1), Cl(m) = 7.09+/-1.37 mL x min(-1), AUC(DOR) = 3602+/-154 microg x min x L(-1)). Considering that both cyclophosphamide and dexamethasone belong to the most widely used immunosuppressive drugs, their potential to promote the CYP2D-mediated metabolism might have a clinical impact in combined therapy of autoimmune or other diseases.

Alterations in N-acetylation of 3-methylhistidine in endotoxemic parenterally fed rats

N-methylhistidine (3-meH) is endogenously released during muscle catabolism and serves as a marker of protein turnover. In rats > 85% of 3-meH is excreted in the urine as the N-acetyl derivative. It has been reported that the percent of non-acetylated 3-meH (NA-3-meH) varies minimally with stress. To further evaluate these reports we randomized 39 male Sprague-Dawley rats (157-213 g) to receive parenteral nutrition only (PN) or PN plus continuous infusion of Escherichia coli 026:B6 lipopolysaccharide at 6 (LPS-6) or 12 (LPS-12) mg.kg-1.d-1 for 48 h. All animals received isocaloric and isonitrogenous PN 24 h before and throughout the study with water ad libitum. Total 3-meH excretion was significantly increased (P < 0.05) in the LPS-6 (470 +/- 136 micrograms/48 h) and LPS-12 (557 +/- 171 micrograms/48 h) groups versus the PN (331 +/- 126 micrograms/48 h) group. NA-3-meH differed significantly between the LPS-12 (218 /+- 89 micrograms/48 h, LPS-6 (94 +/- 48 micrograms/48 h), and PN (39 +/- 12 micrograms/48 h) groups (P < 0.05). Percent NA-3-meH increased significantly from 12.7 +/- 3.9% in the PN group to 19.8 +/- 8.0 and 39.9 +/- 12.8% in the LPS-6 and LPS-12 groups, respectively (P < 0.05). No significant changes in acetyl 3-meH were found between groups. These data suggest that either saturation or inhibition of acetylation pathways occurs with increasing levels of stress. Due to the disproportionate increases in NA-3-meH and percent NA-3-meH during endotoxemia, only total 3-meH should be used as an indicator of protein turnover in rats.

Effect of streptolysin-O-on rat hepatic acetyl coenzyme-A: arylamine N-acetyltransferase and cytochrome P-450 2B 1/2 activities ex vivo

Numerous immunostimulants have been found to increase N-acetylation in vivo but are not associated with a similar increase in vitro. Streptolysin-O (SLO), a thiol-activated (oxygen-labile) hemolytic and immune-stimulating exotoxin produced by group A streptococci, has been reported to increase the metabolic rate constant for sulfamethazine in vivo and arylamine N-acetyltransferase (NAT) activity toward procainamide (PA) ex vivo. The effect of SLO pretreatment of rats on cytochrome P-450-catalyzed tolbutamide hydroxylation and NAT activities toward PA (a substrate for NAT1), and p-aminobenzoic acid (a substrate for NAT2) was examined ex vivo. Subacute SLO (SIGMA Chemical Company, St. Louis, MO) pretreatment (100 Hemolytic Units/kg/day, intraperitoneal, for 4 days) did not alter body weight, liver weight or cytosolic protein content as compared with controls. SLO-pretreatment did not alter NAT activities measured ex vivo, nor was an alteration in tolbutamide hydroxylation observed. Pretreatment with an alternative SLO preparation (DIFCO Laboratories, Detroit, MI) also failed to alter the parameters of body weight, liver weight or cytosolic protein content as compared with controls. While treated animals had significantly reduced microsomal protein content, SLO pretreatment failed to alter the enzyme activities measured. We conclude that SLO does not serve as a useful model immunostimulant for mechanistic studies as it produces no consistent effect on drug metabolizing enzymes.

Effects of interferon-gamma and streptolysin O on hepatic procainamide N-acetyltransferase and various microsomal cytochrome P450-dependent monooxygenases in rats

Immunostimulants known to initiate cytokine production were found to inhibit processes of microsomal drug oxidation but to activate arylamine N-acetylation. The present study investigated the effects of immunstimulating doses of rat interferon-gamma (IFN gamma, 670,000 units ip) and streptolysin O (SLO, 100 HU/kg iv for 5 days) on hepatic cytosolic N-acetyltransferase (NAT) and microsomal cytochrome P450 (CYP)-dependent monooxygenases in male Wistar rats. Both IFN gamma and SLO activated NAT to 120% (P < 0.05) and 135% (P < 0.05), respectively. As expected, monooxygenases were depressed by IFN gamma (P < 0.05) and SLO, the ethylresorufin O-deethylase being the most susceptible enzyme. The results suggested that not only the toxin of gram-positive streptococcal bacteria SLO, but also the cytokine IFN gamma can stimulate NAT activity in rat hepatic cytosol. While the enhancing SLO effect on NAT could not be neutralized by the inhibitor of transcription actinomycin D, NAT stimulation by IFN gamma was abolished by actinomycin D and by the inhibitor of translation, cycloheximide. Obviously, SLO activated NAT independent of protein synthesis and different from IFN gamma-mediated pathways. Posttranslational processes might be involved in NAT stimulation in the rats.

Effects of streptolysin O, picibanil (OK 432) and interferon alpha 2A on cytochrome P-450-dependent monooxygenases and arylamine N-acetyltransferase in rat liver

Streptolysin O, a thiol-activated exotoxin from group A beta-haemolytic streptococci, caused a dose-dependent depression of aniline hydroxylase, aminopyrine N-demethylase and ethylmorphine N-demethylase activities when added into the hepatic microsomal mixtures from male rats at concentrations 0.02-0.4 HU/mL in vitro. The activities of 7-ethoxycoumarin O-deethylase, 7-ethylresorufin O-deethylase and 7-pentylresorufin O-depentylase were not altered with the used concentrations of the toxin. Specific antibody against haemolytic action of streptolysin O added to incubation mixtures in vitro was not able to protect streptolysin-sensitive monooxygenases from the inhibition. The addition of streptolysin O (0.01-0.8 HU/mL) into the cytosol-containing medium did not significantly influence the activity of procainamide N-acetyltransferase. Immunomodulators picibanil (OK 432) and human recombinant interferon alpha 2A which are known to suppress oxidative metabolism in vivo in humans and animals, were without effect either on the cytochrome P-450-dependent monooxygenases or on the N-acetyltransferase activity when administered in vitro at the doses real in their clinical application (0.001-0.1 KE/mL of picibanil and 10-500 U/mL of alpha-interferon).