In homogenates of isolated hepatocytes separated by Nycodenz density gradient centrifugation, two peroxisomal populations are identified that differ in buoyant density. Organelles present in a high density fraction (1.22–1.23 g/cm3) represent mature peroxisomes. Vesicles of intermediate density (1.16–1.17 gm̧3) represent mature peroxisomes. Vesicles of intermediate density (1.16–1.17 g/cm3) are present in much lower concentration and seem to play a particular role in the import and distribution or newly synthesized peroxisomal proteins. In a typical pulse-chase experiment with a 7.5 min pulse, peroxisomal acyl-CoA oxidase is first imported into the peroxisomal fraction of intermediate density. After a chase of up to 30 min, the enzyme is found in mature peroxisomes.
