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 [55Fe]heme with PBR. Transfection of the cloned mouse PBR-isoquinoline carboxamide-binding protein (PBR/IBP) cDNA into monkey kidney Cos-1 cells resulted in a 2.5-fold increase in [55Fe]hemin binding sites, concomitant with the increase in [3H]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 Kd 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 [55Fe]hemin, the binding to PBR had a Kd of 15 +/- 1.8 nM. The Bmax of [55Fe]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.
Taketani
S
S
Department of Hygiene, Kansai Medical University, Osaka.
Kohno
H
H
Furukawa
T
T
Tokunaga
R
R
D000818:Animals; D001665:Binding Sites; D001692:Biological Transport; D002352:Carrier Proteins / Q000737:chemistry / Q000235:genetics; D002478:Cells, Cultured; D002522:Chlorocebus aethiops; D006418:Heme / Q000378:metabolism* / Q000493:pharmacokinetics; D006427:Hemin / Q000378:metabolism; D007424:Intracellular Fluid / Q000378:metabolism; D007501:Iron / Q000378:metabolism; D007504:Iron Radioisotopes; D007668:Kidney / Q000378:metabolism / Q000502:physiology; D051379:Mice; D008930:Mitochondria, Liver / Q000378:metabolism; D011166:Porphyrins / Q000378:metabolism* / Q000493:pharmacokinetics; D011524:Protoporphyrins / Q000378:metabolism; D011963:Receptors, GABA-A / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D014162:Transfection
D002352:Carrier Proteins; D007504:Iron Radioisotopes; D011166:Porphyrins; D011524:Protoporphyrins; D011963:Receptors, GABA-A; C069997:Tspo protein, rat; D006418:Heme; C011539:protoporphyrinogen; D006427:Hemin; D007501:Iron