Metabolism of leukotrienes.

The in vitro metabolism of leukotriene B4 is initiated by omega-hydroxylation. This reaction is followed by oxidation of the omega-hydroxyl group to a carboxyl group. In vivo extensive beta-oxidation occurs and the main excreted products after administration of leukotriene B4 are water and carbon dioxide. Experiments performed in vitro and in vivo have demonstrated that a major pathway of metabolism of the glutathione containing leukotrienes involves modifications of the tripeptide substituent. The metabolic alterations are initiated by enzymatic elimination of the N-terminal gamma-glutamyl residue, catalyzed by the enzyme gamma-glutamyl transferase. This reaction is followed by hydrolysis of the remaining peptide bond resulting in elimination of the C-terminal glycine residue. The enzyme catalyzing the latter reaction is a membrane bound dipeptidase which occurs in kidney and other tissues. The product formed by these reactions, leukotriene E4, has been tentatively identified as a urinary metabolite in man following intravenous administration of leukotriene C4. In rats, the two major fecal metabolities of leukotriene C4 were characterized as being N-acetyl leukotriene E4 and N-acetyl 11-trans leukotriene E4. These compounds are formed in reactions between leukotriene E4 or 11-trans leukotriene E4 and acetyl coenzyme A. The reactions are catalyzed by a membrane bound enzyme present in liver, kidney and other tissues.