Stimulation of Ca 2+ release from rat liver mitochondria by the dithiol reagent α-lipoic acid

Abstract

Rat liver mitochondria contain a Ca 2+-specific release pathway stimulated by Ca 2+-dependent hydrolysis of oxidized intramitochondrial pyridine nucleotides to ADP ribose and nicotinamide. We have previously shown that NAD + hydrolysis and subsequent Ca 2+ release are inhibited by cyclosporine A and that they are only possible when some critical thiols are cross-linked or oxidized, e.g. by phenylarsine oxide, gliotoxin, or peroxynitrite. We now report that the antioxidant α-lipoic acid stimulates Ca 2+ release from intact mitochondria, i.e. with preservation of the mitochondrial membrane potential and without large-amplitude swelling. The release stimulated by α-lipoic acid is inhibited by cyclosporine A and is more effective when the pyridine nucleotides are oxidized. The results strongly suggest that α-lipoic acid stimulates the Ca 2+-specific release pathway from intact mitochondria by oxidizing some vicinal thiols, thereby stimulating hydrolysis of oxidized pyridine nucleotides. These observations further corroborate that intact rat liver mitochondria contain a specific Ca 2+ release pathway stimulated by modification of vicinal thiols. Prolonged stimulation of Ca 2+ release by lipoic acid followed by its re-uptake (Ca 2+ “cycling”) may contribute to the detrimental, prooxidant-like effects seen with higher concentrations of lipoic acid.