Halothane metabolism. Impairment of hepatic omega-oxidation of leukotrienes in vivo and in vitro.

Abstract

Omega-oxidation of leukotrienes is the initial step of hepatic degradation and thus inactivation of these proinflammatory mediators. Omega-oxidation is followed by beta-oxidation of leukotrienes from the omega-end. After exposure of rats to a single dose of the anesthetic agent halothane, a transient decrease in leukotriene omega-oxidation was induced both in vivo and in vitro. In untreated rats, 44.1 +/- 6.0% of N-[3H]acetylleukotriene E4 injected intravenously was recovered unchanged in bile collected for 60 min in vivo; 46.5 +/- 3.0% was recovered as omega-/beta-oxidation products, of which 24.7 +/- 4.5% were associated with beta-oxidation products only (mean +/- SEM; n = 5). In rats receiving a single dose of halothane 18 h before the experiment, recovery of unchanged N-[3H]acetylleukotriene E4 was significantly increased to 79.8 +/- 4.8%, while the fraction of omega-/beta-oxidation products decreased to 9.0 +/- 1.7% (n = 5); 90 h after exposure to halothane, N-[3H]acetylleukotriene E4 recovery decreased to 30.0 +/- 3.0% and omega-/beta-oxidation products amounted to 49.1 +/- 3.8%; the fraction of beta-oxidation products was significantly increased to 43.1 +/- 3.4% (n = 5). Ten days after exposure of rats to halothane, the recoveries of N-[3H]acetylleukotriene E4, of omega-/beta-oxidation products, and of beta-oxidation products alone, returned to almost normal values. Microsomal fractions obtained from rat hepatocytes catalyzed the NADPH- and O2-dependent leukotriene omega-oxidation in vitro. The formation of omega-hydroxy-metabolites of leukotriene B4, leukotriene E4, and N-acetylleukotriene E4 was decreased by 50% in microsomal fractions obtained from rats 18 h and 90 h after halothane treatment, and returned back to control levels in microsomal fractions obtained 10 days after halothane treatment. The Km value of leukotriene B4 omega-oxidation revealed no significant change in enzyme affinity towards leukotriene B4; in contrast, as reflected by the reduction of the Vmax value by 65%, a decrease in the amount of the active enzyme in microsomes obtained from rats 18 h after halothane treatment was observed. Halothane-metabolism-dependent trifluoroacetylation of hepatic proteins may mediate this process. Thus, the time course of the density on immunoblots of trifluoroacetylated protein adducts paralleled that of the transient decrease in leukotriene omega-oxidation. In contrast to its omega-oxidation, leukotriene B4 synthesis from 5-hydroperoxyeicosatetraenoate was not inhibited in hepatocyte homogenates obtained from rats pretreated with halothane. The data suggest that metabolism of halothane causes a transient derangement of hepatic leukotriene homeostasis in vivo.