BIOGENESIS OF PEROXISOMES IN REGENERATING RAT LIVER

II. IMMUNOELECTRON MICROSCOPICAL INVESTIGATION OF CATALASE AND ACYL-CoA OXIDASE

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The intracellular localization of two peroxisomal enzymes, catalase and acyl-CoA oxidase, in regenerating rat liver after partial hepatectomy has been investigated by means of postembedding protein A-gold immunocytochemical technique. The peroxisomes in regenerating rat liver underwent marked changes in shape and size as revealed by catalase cytochemistry with DAB. By immunoelectron microscopy, all peroxisomes, irrespective of size and configuration were strongly positive for catalase showing numerous gold particles distributed over their matrix sparing the electron dense nucleoid region. Sections incubated with the monospecific antibody against acyl-CoA oxidase showed essentially a similar distribution of gold particles, although the labeling density was weaker than with catalase. In all sections the endoplasmic reticulum and the Golgi complex were negative for both catalase and acyl-CoA oxidase, thus ruling out any involvement of the secretory apparatus in the biosynthesis of those proteins. The uniform distribution of catalase and acyl-CoA oxidase in all peroxisomes (presumably new and old) is consistent with the concept of transfer of matrix proteins from preexisting particles to new ones.
In addition to the localization of catalase in the matrix of peroxisomes, some gold particle aggregates were also noted apparently on ribosomes in the cytoplasmic matrix. Serial section analysis, however, revealed that all cytoplasmic labeling was due to tangentially sectioned peroxisomes, especially those with abnormal shapes. Our observations suggest that the concentration of nascent peroxisomal proteins in the cytoplasm of rat liver cells under physiological conditions is too low to be detectable by the presently available immunocytochemical techniques.