Studies of cytochrome synthesis in rat liver

Abstract

The incorporation of radioactive amino acids and of δ-amino[2,3-3H2]laevulinate into rat liver cytochromes b5 and c and cytochrome oxidase has been examined with and without protein-synthesis inhibitors. Cycloheximide promptly inhibits labelling of both haem and protein for cytochrome c in parallel fashion. Although incorporation of 14C-labelled amino acid into microsomal cytochrome b5 is also rapidly inhibited, cycloheximide incompletely inhibits haem labelling of cytochrome b5 and cytochrome a+a3, and inhibition occurs only after repeated antibiotic injections. The possibility of apo-protein pools, or of haem exchange, with a rapidly renewed ‘free’ haem pool, is considered. Consistent with this model is the observation of non-enzymic haem exchange in vitro between cytochrome b5 and methaemoglobin. Chloramphenicol, injected intravenously over 5h, results in a 20–40% decrease in incorporation of δ-amino[2,3-3H2]laevulinate into haem a+a3 and haem of cytochromes b5 and c. With the dosage schedule of chloramphenicol studied, amino acid labelling of total liver protein and of cytochrome c was not inhibited. Similarly, ferrochelatase activity was not decreased.