Porphyrins are endogenous ligands for the mitochondrial (peripheral-type) benzodiazepine receptor.

Abstract

ABSTRACT"Peripheral-type" benzodiazepine receptors are localized to the outer mitochondrial membrane. We have identified potent competitive inhibitors of these receptors and purified them from human blood and from several rat organs. TLC analysis of the purified inhibitor from erythrocytes displays a single peak of inhibitory activity with an absorbance spectrum identical to hemin. All of the inhibitory activity in extracts of several tissues can be accounted for by their porphyrin and metalloporphyrin content. Pure hemin and protoporphyrin IX competitively inhibit mitochondrial benzodiazepine binding with K1 values of 41 and 15 nM, respectively, and are less active by a factor of 1000 at central-type benzodiazepine receptors. Thus, porphyrins appear to be endogenous ligands for mitochondrial benzodiazepine receptors.The anxiolytic effects of benzodiazepines are mediated by a "central" benzodiazepine receptor, located primarily in the brain. However, many benzodiazepines bind with high affinity to a "peripheral-type" receptor found in numerous organs whose drug specificity differs from the central-type receptor (1). The characteristic localization of this site in tissues, such as the zona glomerulosa of the adrenal gland, the testosterone-forming Leydig cells of the testes, the olfactory nerves of the brain, and the distal tubules and collecting ducts of the kidney, probably reflects a unique functional role for the receptor (1). Drugs selective for peripheral-type benzodiazepine receptors produce a pleiotropic spectrum of pharmacologic actions, including stimulation or inhibition of cell proliferation (2-4), alteration of immune function (5, 6), alteration in cardiac action potentials (7-9), effects on convulsive threshold (10-12), and alteration of protooncogene expression (13). Our subcellular localization studies indicate that the receptor is selectively associated with the outer membrane of mitochondria; hence it may be properly referred to as the "mitochondrial benzodiazepine receptor" (14, 15).To gain insight into the physiological role of the mitochondrial benzodiazepine receptor, several laboratories have examined tissue extracts for a possible endogenous ligand. High (16) and low (16-20) molecular weight inhibitory activities have been reported in blood (17, 18), urine (17-20), and various organ extracts (16). The chemical identity has not been established for any of these inhibitory activities. In the present study we demonstrate that the low molecular weight substances in extracts of numerous tissues that compete for mitochondrial benzodiazepine receptor binding are porphyrins and that porphyrins have nanomolar affinity for the receptor.MATERIALS AND METHODS Materials. Porphyrins were obtained from Porphyrin Products (Logan, UT) and their purity was monitored by HPLC. All other materials were provided by standard sources. Benzodiazepines {7-chloro-1,3-dihydro-1-methyl-5-(p-chlorophenyl)-2H-1,4-benzodiazepine-2-one (Ro 5-4864); ethyl 8-fluoro-5 ,6-dihydro-5-methyl-6-oxo-4H-imidazo[1 ,5-a][1, 4]benzodi...

Mitochondrial Release of Apoptosis-Inducing Factor and Cytochrome c During Smooth Muscle Cell Apoptosis

“…In support of the latter concept, porphyrin-derived photosensitizers may localize to and directly bind mitochondrial proteins such as the peripheral benzodiazepine receptor or the adenine nucleotide translocator. 35,36 Thus, it is most likely that PDT-induced SMC apoptosis is triggered at or near mitochondria. Further evidence of PDT acting on mitochondria is provided by the presence of the mitochondrial 7A6 antigen after PDT and release of AIF.…”

Porphyrin Accumulation in Mitochondria Is Mediated by 2-Oxoglutarate Carrier

“…Heme is also utilized in mitochondria for the synthesis of heme proteins. It has been reported that hemin and protoporphyrin IX bind to the peripheral-type benzodiazepine receptor (25,26) located in the mitochondrial outer membrane. As OGC is well conserved in yeast and human cells and localizes to the mitochondrial inner membrane (27), it may play an important role in the accumulation of endogenous porphyrin heme and the heme precursors in mitochondria.…”

Transient protection by peripheral benzodiazepine receptors during the early events of ultraviolet light-induced apoptosis

“…1,2 Endogenous ligands for PBRs include protoporphyrin IX and a newly described cytoplasmic protein, PRAX-1, that specifically interacts with these receptors. 3,4 PBRs are involved in steroidogenesis, heme biosynthesis, immune and stress responses, cell growth, differentiation and mitochondrial respiratory control. 5 Recent data also suggest that PBR is structurally and functionally related to the Rhodobacter sphaeroides tryptophan-rich sensory protein (TspO).…”