It has been demonstrated that leukotriene (LT)C4 is metabolized to LTD4 via the action of the enzyme gamma-glutamyl transpeptidase. LTD4 is, in turn, converted by the enzyme aminopeptidase to LTE4. In the present study, the pharmacological effects of the aminopeptidase inhibitor, L-cysteine and the gamma-glutamyl transpeptidase inhibitor, L-serine borate, on peptide LT concentration-response curves were evaluated in isolated guinea-pig trachea. L-Cysteine (3 mM) enhanced the contractile activity of both LTC4 and LTD4. L-Serine borate (45 mM) enhanced the contractile activity of LTC4 without altering the response to LTD4. In contrast, neither L-cysteine nor L-serine borate consistently altered the concentration-response curves to LTE4, histamine or carbachol, which rules out a nonspecific effect of these inhibitors on airway smooth muscle. In the absence of enzyme inhibitors the peptide LTs were equipotent; whereas in their presence the relative order of potency was LTC4 = LTD4 greater than LTE4. Incubation of isolated guinea-pig trachea with [3H]LTC4 resulted in the formation of [3H]LTD4 and [3H]LTE4 with a proportional decrease in [3H]LTC4. The bioconversion of [3H]LTC4 was blocked by L-serine borate in a concentration-related manner (IC50 = 3.4 +/- 0.5 mM, mean +/- S.E.M., n = 3) and the formation of LTE4 was blocked by L-cysteine (10 mM). The results suggest that LTC4 is converted to LTD4 and subsequently to LTE4 by isolated guinea-pig trachea. The potency of LTC4 and LTD4 is increased when their transformation to LTE4 is prevented.