Matrix free Mg(2+) and the regulation of mitochondrial volume.
Mitochondria must maintain volume homeostasis in order to carry out oxidative phosphorylation. It has been postulated that the concentration of free Mg(2+) ([Mg(2+)]) serves as the sensor of matrix volume and regulates a K(+)-extruding K(+)/H(+) antiport (K. D. Garlid. J. Biol. Chem. 255: 11273-11279, 1980). To test this hypothesis, the fluorescent probe furaptra was used to monitor [Mg(2+)] and free Ca(2+) concentration ([Ca(2+)]) in the matrix of isolated beef heart mitochondria, and K(+)/H(+) antiport activity was measured by passive swelling in potassium acetate. Concentrations that result in 50% inhibition of maximum activity of 92 microM matrix [Mg(2+)] and 2.2 microM [Ca(2+)] were determined for the K(+)/H(+) antiport. Untreated mitochondria average 670 microM matrix [Mg(2+)], a value that would permit <1% of maximum K(+)/H(+) antiport activity. Hypotonic swelling results in large decreases in matrix [Mg(2+)], but swelling due to accumulation of acetate salts does not alter [Mg(2+)]. Swelling in phosphate salts decreases matrix [Mg(2+)], but not to levels that permit appreciable antiport activity. We conclude that 1) it is unlikely that matrix [Mg(2+)] serves as the mitochondrial volume sensor, 2) if K(+)/H(+) antiport functions as a volume control transporter, it is probably regulated by factors other than [Mg(2+)], and 3) alternative mechanisms for mitochondrial volume control should be considered.
Jung
D W
DW
Department of Medical Biochemistry, College of Medicine, The Ohio State University, Columbus, Ohio 43210, USA. jung.7@osu.edu
Brierley
G P
GP
D000818:Animals; D017920:Antiporters / Q000378:metabolism; D002118:Calcium / Q000378:metabolism / Q000494:pharmacology; D002417:Cattle; D005456:Fluorescent Dyes; D016257:Fura-2 / Q000031:analogs & derivatives; D006859:Hydrogen / Q000378:metabolism; D008274:Magnesium / Q000378:metabolism* / Q000494:pharmacology; D008928:Mitochondria / Q000737:chemistry / Q000502:physiology*; D008933:Mitochondrial Swelling / Q000502:physiology*; D009206:Myocardium; D011188:Potassium / Q000378:metabolism; D027961:Potassium-Hydrogen Antiporters; D014882:Water-Electrolyte Balance / Q000502:physiology
D017920:Antiporters; D005456:Fluorescent Dyes; D027961:Potassium-Hydrogen Antiporters; C058776:2-(2-(5-carboxy)oxazole)-5-hydroxy-6-aminobenzofuran-N,N,O-triacetic acid; D006859:Hydrogen; D008274:Magnesium; D011188:Potassium; D002118:Calcium; D016257:Fura-2