A general characteristic of tumor mitochondria: Leakage of endogenous Mg2+ on incubation with uncoupler and resultant reduction of uncoupler-stimulated ATPase activity

Previous studies showed that stimulation of mouse mitochondrial ATPase activity of tumor cells, fetal liver, and adult brain by the uncoupler 2,4-dinitrophenol was markedly suppressed during incubation of the mitochondria with the uncoupler (J.-I. Hayashi et al., 1980, Biochem. Biophys. Res. Commun.92, 261–267). The present work showed the reason for this suppression. More than half the endogenous Mg²⁺ leaked from mitochondria of all tumor cells tested, and of fetal liver and adult brain during incubation with the uncoupler, while only about 30% of the endogenous Mg²⁺ leaked from mitochondria of other normal tissues. The effect of the uncoupler on Mg²⁺ leakage from liver mitochondria changed from the fetal to the adult type within about 30 min after birth. In hypotonic medium, normal liver mitochondria also lost more than half their total Mg²⁺ and concomitantly stimulation of their ATPase activity by uncoupler was considerably reduced. Exogenously added Mg²⁺ could reverse this reduced effect of the uncoupler on ATPase activity of mitochondria from normal tissues and tumor cells. These results show that the endogenous Mg²⁺ content of mitochondria directly affects the stimulation by uncoupler of ATPase activity of mitochondria from both normal tissues and tumor cells. Thus, mitochondria of all tumor cells tested, and of fetal liver and adult brain are leaky to Mg²⁺ during incubation with uncoupler and as a result of the leakage, the stimulatory effect of the uncoupler on their ATPase activity is greatly reduced.