Localization of uric acid oxidase activity in core and matrix of peroxisomes as detected in unfixed cryostat sections of rat liver.

Because of controversial data in the literature, we studied the localization of uric acid oxidase (UAOX) activity in rat liver by light microscopy (LM) and electron microscopy (EM). UAOX is partially inactivated by aldehyde fixation and therefore we developed a technique that permits the use of unfixed cryostat sections for both LM and EM studies. Sections of rat liver were mounted on a semipermeable membrane stretched over a gelled incubation medium containing urate as specific substrate for UAOX and cerium ions to capture H2O2 produced by oxidase activity. The specificity of the reaction was checked by comparing incubations in the presence of substrate with incubations either in the absence of substrate or in the presence of substrate and 2,6,8-trichloropurine, a competitive inhibitor of UAOX. After incubation the sections were either fixed immediately for EM or visualized for LM with a second-step incubation. At the LM level, final reaction product was found in a granular form, homogeneously distributed throughout the hepatocytes. EM revealed excellent subcellular morphology and electron-dense reaction product in both the core and the matrix of peroxisomes, but not in other organelles or the cytoplasmic matrix. After incubations without substrate or with substrate and inhibitor, hardly any reaction product was found. We conclude that, because of the use of unfixed tissue, UAOX is not inactivated, which results in localization of UAOX activity not only in the core of peroxisomes but also in the peroxisomal matrix.