31P-nuclear magnetic resonance analysis of the intracellular pH in the slime mold Dictyostelium discoideum.

Amoebae of the slime mold D. discoideum were studied by phosphorus nuclear magnetic resonance. Under aerobic conditions, major intracellular phosphate compounds included phosphomonoesters, inorganic phosphate (Pi), ADP and ATP. Nucleotides were essentially as magnesium complexes. Two intracellular Pi resonances were clearly resolved and the corresponding pHs determined by the chemical shifts characteristics were 7.7 and 6.7. These intracellular pHs were strictly constant over an extracellular pH range between 5.0 and 7.5. The two cellular compartments defined by the Pi resonances were assigned to mitochondria (pH 7.7) and cytosol (pH 6.7) on the basis of their response to anaerobiosis or to carbonylcyanide-m-chloro phenylhydrazone (CCCP), an uncoupler of oxidative phosphorylation, which equilibrate the two intracellular pHs.