Biogenesis of peroxisomes: Intracellular site of synthesis of catalase and uricase (rat liver/free and membrane-bound polysomes/cell-free protein synthesis/immunoprecipitation with monospecific antisera

The intracellular site of synthesis of two peroxisomal enzymes of rat liver, uricase (urate:oxygen oxidoreductase, EC 1.7.3.3) and catalase (hydrogen peroxide:hydrogen peroxide oxidoreductase, EC 1.11.1.6), has been localized on free ribosomes and not membrane-bound ribosomes. Free polysomes and membrane-bound polysomes, prepared by classical cell fractionation techniques from rat liver, were incubated for protein synthesis in a cell-free system derived from rabbit reticulocytes. Characterization of the total translation products by polyacrylamide gel electrophoresis in sodium dodecyl sulfate, as well as by immunoprecipitation with anti-rat albumin antiserum, confirmed that good separation of the two polysome classes was achieved. Uricase and catalase were immunoprecipitable from translation products directed by free polysomes orpbeolextracted free polysomal mRNA but not from products of Iembrane-bound polysomes. Furthermore, unlike albumin, nascent uricase and catalase were not cotranslationally segregated by dog pancreas microsomal membranes. The results indicate that uricase and catalase are transferred to the interior of peroxisomes by a post-translational mechanism; an hypothesis is ormulated here or e biogenesis of peroxisomes. Several laboratories have attempted to determine the intracellular site of synthesis of catalase (hydrogen peroxide:hydrogen peroxide oxidoreductase, EC 1.11.1.6) by various approaches (for review, see ref. 1). However, none of the reported results has provided clear-cut evidence as to whether catalase is synthesized on free ribosomes, on membrane-bound ribosomes, or on both. In the present study we have investigated the intracellular site of synthesis of two peroxisomal enzymes of rat liver, catalase and uricase (urate:oxygen oxidoreductase, EC 1.7.3.3). Our data demonstrate that both peroxisomal enzymes are synthesized exclusively by free ribosomes. The implications of these results for the segregation of peroxisomal proteins within peroxisomes are discussed, and an hypothesis for the biogenesis of peroxi-