Latexin, a carboxypeptidase A inhibitor, is expressed in rat peritoneal mast cells and is associated with granular structures distinct from secretorygranules and lysosomes

Latexin, a protein possessing inhibitory activity against rat carboxypeptid ase Al (CPA1) and CPA2, is expressed in a neuronal subset in the cerebral c ortex and cells in other neural and non-neural tissues of rat. Although lat exin also inhibits mast-cell CPA (MCCPA), the expression of latexin in rat mast cells has not previously been confirmed. In the present study we exami ned the expression and subcellular localization of latexin in rat peritonea l mast cells. Western blot and reverse-transcriptase-mediated PCR analyses showed that latexin was contained and expressed in the rat peritoneal mast cells, Immunocytochemically, latexin immunofluorescence was localized on gr anular structures distinct from MCCPA-, histamine- or cathepsin D-immunopos itive granules. Immunoelectron microscopy revealed that latexin was associa ted with a minority population of granules. The latexin-associated granules were separated from MCCPA- or histamine-containing granules on a self-gene rating density gradient of polyvinylpyrrolidone-coated silica-gel particles (Percoll). Treatments with high ionic strength and heparinase released lat exin from the granules, suggesting that latexin is noncovalently associated with a heparin-like component of the granules. MCCPA and histamine were re leased from the mast cells after non-immunological and immunological stimul ation with compound 48/80, A23187 and anti-IgE antibody, whereas latexin wa s not released. These results show that latexin is synthesized in rat perit oneal mast cells and suggest that it is associated with a unique type of in tracellular granules distinct from MCCPA- and histamine-containing secretor y granules and lysosomes.