CYTOPLASMIC AND PEROXISOMAL CATALASES OF THE GUINEA-PIG LIVER - EVIDENCE FOR 2 DISTINCT PROTEINS

Catalase, a peroxisomal marker enzyme in the liver of most mammals, is found by immune-electron microscopy in guinea pig (GP) hepatocytes no t only in peroxisomes, but also in the cytoplasm (Beier et al. (1988) fur. J. Cell Biol. 46, 129-135). We have been able to distinguish in G P liver homogenates between the cytosolic catalase and that part of th e enzyme activity which is due to leakage of the enzyme from peroxisom es by adding 4% polyethylene glycol to the homogenization medium. This approach revealed that approximately 40% of the total catalase activi ty and almost all of alpha-hydroxy-acid oxidases are peroxisomal, whil e 60% of catalase is of genuine cytosolic origin. The cytosolic and pe roxisomal catalases of guinea pig were purified to homogeneity and wer e analyzed by SDS-PAGE and isoelectric focussing. The cytosolic catala se exhibited a slightly higher M(r) (approximate to 1000) and a less a cidic pi than the peroxisomal enzyme. Limited proteolysis and amino-ac id analysis revealed also slight differences between the two molecular forms of catalase. Total RNA was isolated from guinea pig liver and t ranslated in vitro by using a rabbit reticulocyte lysate system. Immun oprecipitation with an antibody against guinea pig catalase followed b y high-resolution polyacrylamide gel electrophoresis revealed two poly peptide bands differing slightly in M(r). These observations suggest s trongly, that cytoplasmic and peroxisomal catalases in guinea pig live r are two ap closely related but distinct proteins.