The subcellular distribution of the biosynthetic intermediates of catalase was studied in the livers of rats receiving a mixture of [3H]leucine and [14C]δ-aminolevulinic acid by intraportal injection. Postnuclear supernates were fractionated by a one-step gradient centrifugation technique that separates the main subcellular organelles, partly on the basis of size, and partly on the basis of density. Labeled catalase and its biosynthetic intermediates were separated from the gradient fractions by immunoprecipitation, and the distributions of radioactivity were compared with those of marker enzymes. The results show that catalase protein is synthesized outside the peroxisomes, but rapidly appears in these particles, mostly still in the form of the first hemeless biosynthetic intermediate. Addition of heme and completion of the catalase molecule take place within the peroxisomes. During the first 15 min after [3H]leucine administration, more than half of the newly formed first intermediate was recovered in the supernatant fraction, where it was found to exist as an aposubunit of about 60,000 molecular weight.

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