CA2-MEMBRANE CAN STIMULATE MITOCHONDRIAL PERMEABILITY TRANSITION INDUCED BY PHENYLARSINE OXIDE( ACTING AT THE EXTERNAL SIDE OF THE INNER MITOCHONDRIAL)

Mitochondrial permeability transition (MPT) induced by the thiol cross -linker phenylarsine oxide (PhAsO) in Ca2+-depleted mitochondria incub ated in the presence of ruthenium red, an inhibitor of the Ca2+ unipor ter, is stimulated by the addition of extramitochondrial Ca2+. The pre sence of extramitochondrial Ca2+ stimulates the reaction of mitochondr ial membrane protein thiol groups with PhAsO. Both Ca2+-induced increa se in mitochondrial membrane permeabilization and protein thiol group reaction with PhAsO are dependent on time (5-10min to be complete) and the concentration of Ca2+ (1-25 mu M). Mitochondrial permeabilization induced by PhAsO (15 mu M) and extramitochondrial Ca2+ is inhibited b y ADP, cyclosporin A, dibucaine and Mg2+, while mitochondrial permeabi lization induced by high concentrations of PhAsO (60 mu M) in the abse nce of Ca2+ is inhibited only by ADP and cyclosporin A. These results suggest that dibucaine and Mg2+ can inhibit mitochondrial permeabiliza tion by antagonizing the effect of Ca2+ on the mitochondrial membrane. Once mitochondrial permeabilization induced by 15 mu M PhAsO and extr amitochondrial Ca2+ has already occurred, the addition of the Ca2+ che lator EGTA restores mitochondrial membrane potential (MPT pore closure ), suggesting that the presence of Ca2+ is essential for the maintenan ce of the permeability of the mitochondrial membrane to protons (MPT p ore opening). In conclusion, the results presented indicate that low C a2+ concentrations acting at the external side of the inner mitochondr ial membrane can stimulate mitochondrial permeability transition induc ed by PhAsO, due to increased accessibility of protein thiol groups to the reaction with PhAsO and increased probability of MPT pore opening . (C) 1997 Elsevier Science B.V.