title: On the relationship between matrix free Mg2+ concentration and total Mg2+ in heart mitochondria.
abstract: The matrix free magnesium ion concentration, [Mg2+]m, estimated using the fluorescent probe furaptra, averaged 0.67 mM in 15 preparations of beef heart mitochondria containing an average of 21 nmol total Mg2+ per mg protein. [Mg2+]m was compared with total Mg2+ during respiration-dependent uptake and efflux of Mg2+ and during osmotic swelling. In the absence of external Pi these mitochondria contain about 32 nmol/mg non-diffusible Mg-binding sites with an apparent Kd of 0.34 mM. [Mg2+]m depends on both the size of the total Mg2+ pool and the ability of matrix anions to provide Mg-ligands. Pi interacts strongly with Mg2+ to decrease [Mg2+]m and, in the absence of external Mg2+, promotes respiration-dependent Mg2+ efflux and a decrease in [Mg2+]m to very low levels. The uptake of Pi by respiring mitochondria converts delta pH to membrane potential (delta psi) and provides additional Mg-binding sites. This permits large accumulations of Mg2+ and Pi with little change in [Mg2+]m. Nigericin also converts delta pH to delta psi in respiring mitochondria and induces a large and rapid increase in both total Mg2+ and [Mg2+]m. Mersalyl increases the permeability of the mitochondrial membrane to cations and this also induces a marked increase in both total Mg2+ and [Mg2+]m. These results suggest that mitochondria take up Mg2+ by electrophoretic flux through membrane leak pathways, rather than via a specific Mg2+ transporter. Mitochondria swollen by respiration dependent uptake of potassium phosphate show decreased [Mg2+]m, whereas those swollen to the same extent in potassium acetate do not. This suggests that [Mg2+]m is well-buffered during osmotic volume changes unless there is also a change in ligand availability.
authors.affiliation: Department of Medical Biochemistry, Ohio State University, Columbus 43210-1218, USA.
authors.forename: D W
authors.initials: DW
authors.lastname: Jung
authors.forename: E
authors.initials: E
authors.lastname: Panzeter
authors.forename: K
authors.initials: K
authors.lastname: Baysal
authors.forename: G P
authors.initials: GP
authors.lastname: Brierley
mesh_terms: D000818:Animals; D001665:Binding Sites; D001692:Biological Transport; D002417:Cattle; D019069:Cell Respiration; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005456:Fluorescent Dyes; D016257:Fura-2 / Q000031:analogs & derivatives; D007476:Ionophores / Q000494:pharmacology; D008274:Magnesium / Q000032:analysis / Q000378:metabolism*; D008634:Mersalyl / Q000494:pharmacology; D008929:Mitochondria, Heart / Q000737:chemistry / Q000378:metabolism*; D008933:Mitochondrial Swelling; D009550:Nigericin / Q000494:pharmacology; D010710:Phosphates / Q000378:metabolism / Q000494:pharmacology; D013050:Spectrometry, Fluorescence
chemical_list: D004791:Enzyme Inhibitors; D005456:Fluorescent Dyes; D007476:Ionophores; D010710:Phosphates; C058776:2-(2-(5-carboxy)oxazole)-5-hydroxy-6-aminobenzofuran-N,N,O-triacetic acid; D008634:Mersalyl; D008274:Magnesium; D009550:Nigericin; D016257:Fura-2