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 digestive 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 revealed an immunoreactive polypeptide with a molecular weight similar to the corresponding mammalian hepatic protein. This crossreactive polypeptide of 32 kDa was particle-bound yet not peroxisome-associated. In paraffin sections the antiserum specifically labeled the plasma membrane of the digestive gland epithelial cells and discrete regions within the perinuclear and apical portions of the digestive tubules and duct cells. By electron microscopy gold particles representing antigenic sites were found on the microvilli and the lateral plasma membrane as well as the membranes of the secretory/ endocytic compartments, that is, the Golgi complex, secretory and some endocytic vesicle membranes. Since the peroxisomal UOX-antibody exhibits a comparable immunoreactivity towards a urate-transporter channel protein in rat kidney proximal tubules and has been used for its molecular cloning (Leal-Pinto 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.