INVOLVEMENT OF PERIPHERAL-TYPE BENZODIAZEPINE RECEPTORS IN THE INTRACELLULAR-TRANSPORT OF HEME AND PORPHYRINS

To investigate the involvement of peripheral-type benzodiazepine receptors (PBR) in heme metabolism, we examined the interaction of [Fe-55]heme with PBR. Transfection of the cloned mouse PBR-isoquinoline carboxamide-binding protein (PBR/IBP) cDNA into monkey kidney Cos-l cells resulted in a 2.5-fold increase in [Fe-55]hemin binding sites, concomitant with the increase in [H-3]PK11195 binding sites, as compared with those seen in antisense PBR/IBP cDNA-transfected cells. The binding of hemin to the transfected receptors exhibited a relatively high affinity with a K-d of 12 nM, and was inhibited by several benzodiazepine ligands, including PK11195, Ro 5-4864, diazepam and protoporphyrin IX. When mouse liver mitochondria were incubated with [Fe-55]hemin, the binding to PBR had a K-d of 15+/-1.8 nM. The B-max of [Fe-55]hemin binding to the mitochondria was 6.88+/-0.76 pmol/mg of protein, a value consistent with that of [3H]PK11195 binding, with a lower affinity. Coproporphyrinogen III, a precursor porphyrin produced in the cytosol, is translocated into mitochondria, then is converted to protoporphyrinogen IX; this conversion decreased in the presence of benzodiazepine ligands. To examine whether this decrease was related to a decrease in the binding of coproporphyrinogen to the mitochondria, the effects of benzodiazepines on the binding of coproporphyrinogen were examined. As the binding was dose-dependently inhibited by PK11195, Ro 5-4864, and diazepam, porphyrins are likely to be endogenous ligands for PBR. We propose that PBR play a role in the intracellular transport of porphyrins and heme.