Regulation of Reverse Uniport Activity in Mitochondria by Extramitochondrial Divalent Cations. Dependence on a Soluble Intermembrane Space Component

We previously reported that uncoupling Ca2(+)-loaded mitochondria in the presence of [ethylenebis(oxyethylenenitrilo)]tetraacetic acid (EGTA) produces a partial expression of the permeability transition. From this and related observations, it was proposed that the absence of external free Ca2+ is inhibitory to reverse activity of the Ca2+ uniporter (Igbavboa, U., and Pfeiffer, D.R. (1988) J. Biol. Chem. 263, 1405-1412). By using Sr2(+)-instead of Ca2(+)-loaded mitochondria, the transition is avoided upon treatment with EGTA plus uncoupler, and inhibition of reverse uniport activity can be observed directly. In the presence of physiological Mg2+ concentrations, reverse uniport of Sr2+ is eliminated by external EGTA following a brief period of rapid activity. It is proposed that binding of Mg2+ rather than Sr2+ (Ca2+) at an external site is responsible for the inhibition. Regulation at the external site is modified by the size of the Sr2+ load. EGTA, in the presence of Mg2+, does not inhibit the reverse uniport-dependent release of Sr2+ from mitoplasts. The inhibitory effect can be recovered by adding back the soluble components obtained as the intermembrane space fraction following removal of the outer membrane. The soluble factor could be a regulatory subunit which contains the external cation binding site. Adjustments to uniporter activity due to regulation by the binding site and/or the soluble factor may be slow and may be significant in determining how mitochondria respond to rapid Ca2+ transients in vivo.