URIC-ACID DEGRADING ENZYMES, URATE OXIDASE AND ALLANTOINASE, ARE ASSOCIATED WITH DIFFERENT SUBCELLULAR ORGANELLES IN FROG LIVER AND KIDNEY

On the basis of differential and density gradient centrifugation studi es, the site of the uric acid degrading enzymes, urate oxidase and all antoinase, in amphibia was previously assigned to the hepatic peroxiso mes. Using specific antibodies against frog urate oxidase and allantoi nase, we have undertaken an immunocytochemical study of the localizati on of these two proteins in frog liver and kidney, and demonstrate tha t whereas urate oxidase is present in peroxisomes, allantoinase is loc alized in mitochondria. Urate oxidase and allantoinase were detected b y immunoblot analysis in both frog liver and kidney. The subcellular l ocalization of these two enzymes was ascertained by Protein A-gold imm unocytochemical staining of Lowicryl K(4)M-embedded tissue. Peroxisome s in frog liver parenchymal cells and kidney proximal tubular epitheli um contained a semi-dense subcrystalloid core, which was found to be t he exclusive site of urate oxidase localization. Allantoinase was dete cted within mitochondria, but not in peroxisomes of hepatocytes or pro ximal tubular epithelium. No allantoinase was detected in the mitochon dria of nonhepatic parenchymal cells in liver and of the cells lining the distal convoluted tubules of the kidney. These results demonstrate that, unlike rat kidney peroxisomes which lack urate oxidase, peroxis omes of frog kidney contain this enzyme. Contrary to previous assumpti ons, these studies also clearly establish that urate oxidase and allan toinase, the first two enzymes involved in uric acid degradation, are localized in different subcellular organelles in frog liver and kidney .