The intracellular pathway of newly formed rat liver catalase

[1-¹⁴C]-l-leucine was injected into the femoral vein of male rats. At various times after injection, from 3 to 120 min, the livers were removed and fractionated into various cell components. The intracellular location of radioactive catalase and albumin were compared. Radioactive catalase and albumin both appeared at 5-10 min after injection but in different intracellular locations. Catalase was first located in the soluble cytoplasmic fraction while albumin was found in the rough endoplasmic reticulum. Peak catalase radioactivity did not occur in the rough endoplasmic reticulum until 20-40 min after injection, at which time albumin was already leaving the smooth endoplasmic reticulum. Radioactive catalase did not enter the smooth endoplasmic reticulum. The catalase which sedimented with the rough endoplasmic reticulum could be separated by sucrose gradient centrifugation from microsomal vesicles which contained nascent albumin. The radioactive catalase accumulated with time into a peroxisomal fraction but some catalase of equal specific radioactivity remained in the soluble cytoplasmic fraction even at 120 min after injection.