Leukotrienes (LTs) are 5-lipoxygenase (5-LO)-derived arachidonic metabolites that constitute a potent set of lipid mediators produced by inflammatory cells. Leukotriene A(4), a labile allylic epoxide formed from arachidonic acid by dual 5-LO activity, is the precursor for LTB(4) and LTC(4) synthesis. LTC(4) is further transformed enzymatically by the sequential action of gamma-glutamyltranspeptidase and dipeptidase to LTD(4) and LTE(4), respectively. In this report, we present evidence that bovine pancreatic carboxypeptidase A (CPA), which shares significant sequence homology with CPA in mast cell granules, catalyzes the conversion of LTC(4) to LTF(4) via the hydrolysis of an amide bond. The identity of CPA-catalyzed LTC(4) hydrolysis product as LTF(4) was confirmed by several analytical criteria, including enzymatic conversion to conjugated tetraene by soybean LO, conversion to LTE(4) by gamma-glutamyltranspeptidase, cochromatography with the standard LTF(4) and positive-ion fast-atom bombardment mass spectral analysis. Thus, it appears that the physiological significance of this single-step transformation may point toward a major cellular homeostatic mechanism of metabolizing LTC(4), a potent bronco- and vasoconstrictor, to a less potent form of cysteinyl LTs.