Tightly coupled mitochondria from human early placenta.

1. A procedure for the isolation of tightly coupled mitochondria from human early placenta is described. 2. Mitochondria obtained by this method were able to oxidize Krebs cycle intermediates, pyruvate, glutamate, glutamine, palmitoyl-carnitine, alpha-glycerophosphate and beta-hydroxybutyrate. 3. These mitochondria incubated in the medium containing ethylene diamine tetraacetic acid and bovine serum albumin and no added Mg2+ ions exhibited a high respiratory control and adenosine diphosphate:oxygen (ADP:O) ratios corresponding to the theoretical values for all substrates tested. Addition of Mg2+ ions markedly reduced the respiratory control index and ADP:O ratio. 4. Adenosine triphosphatase (ATPase) activity in the obtained mitochondrial preparation was stimulated about tenfold by Mg2+. Oligomycin inhibited Mg2+-stimulated ATPase activity by about 25 per cent, but completely inhibited this activity in the absence of Mg2+ ions. 5. It is concluded that the effect of Mg2+ ions on the respiratory control and ADP:O ratio reported in this paper is exerted mainly through the Mg/+-stimulated oligomycin-insensitive ATPase activity.