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.