Immunocytochemical localization of a urate oxidase Immunoreactive protein in the plasma membranes and membranes of the secretory/endocytic compartments of digestive gland cells of the mussel Mytilus galloprovincialis

The subcellular compartmentalization of urate oxidase (UOX) in the digestiv e glands of mussels, Mytilus galloprovincialis Lmk, was studied by means of immunoblotting and immunocytochemistry, using an antibody raised in rabbit against rat liver UOX, Western blot analysis of subcellular fractions reve aled an immunoreactive polypeptide with a molecular weight similar to the c orresponding mammalian hepatic protein. This crossreactive polypeptide of 3 2 kDa was particle-bound yet not peroxisome-associated. In paraffin section s the antiserum specifically labeled the plasma membrane of the digestive g land epithelial cells and discrete regions within the perinuclear and apica l portions of the digestive tubules and duct cells. By electron microscopy gold particles representing antigenic sites were found on the microvilli an d the lateral plasma membrane as well as the membranes of the secretory/end ocytic compartments, that is, the Golgi complex, secretory and some endocyt ic vesicle membranes. Since the peroxisomal UOX-antibody exhibits a compara ble immunoreactivity towards a urate-transporter channel protein in rat kid ney proximal tubules and has been used for its molecular cloning (Leal-Pint o et al,, 1997, J. Biol. Chem. 272, 617-625), we suggest that the membrane protein identified in mussel digestive glands could represent a homologous urate-transporter protein.