Carboxypeptidase A-catalyzed direct conversion of leukotriene C₄ to leukotriene F₄

Leukotrienes (LTs) are 5-lipoxygenase (5-LO)-derived arachidonic metabolites that constitute a potent set of lipid mediators produced by inflammatory cells. Leukotriene A₄, a labile allylic epoxide formed from arachidonic acid by dual 5-LO activity, is the precursor for LTB₄ and LTC₄ synthesis. LTC₄ is further transformed enzymatically by the sequential action of γ-glutamyltranspeptidase and dipeptidase to LTD₄ and LTE₄, 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 LTC₄ to LTF₄ via the hydrolysis of an amide bond. The identity of CPA-catalyzed LTC₄ hydrolysis product as LTF₄ was confirmed by several analytical criteria, including enzymatic conversion to conjugated tetraene by soybean LO, conversion to LTE₄ by γ-glutamyltranspeptidase, cochromatography with the standard LTF₄ 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 LTC₄, a potent bronco- and vasoconstrictor, to a less potent form of cysteinyl LTs.