The subcellular localization of newly synthesized cytochrome b5.

The fate of newly synthesized cytochrome b5 was studied in rat hepatocytes. Using an antibody specific for microsomal cytochrome b5, we found newly synthesized microsomal cytochrome b5 in both mitochondria and a mitochondria associated membrane fraction as well as in microsomes. Newly synthesized cytochrome b5 was quickly removed from the site of synthesis on free ribosomes and inserted into membranes at random. No migration of newly synthesized cytochrome b5 between cellular compartments was observed and therefore the assembly of the apoprotein with the heme moiety is apparently not taking place in any particular cellular compartment.