Properties of phosphate bound to cerebral microsomes during adenosine-triphosphatase activity.

1. Ox-brain microsomes were incubated with [gamma-(32)P]ATP under various conditions. After the reaction, which was stopped with trichloroacetic acid, a small amount of phosphate remained bound to the washed precipitate. 2. Properties of the bound phosphate were studied by treatment with buffers and solvents. 3. The Na(+)-dependent increment in bound phosphate, predominant at low ATP concentration and features of which suggest involvement in the concomitant adenosine-triphosphatase activity, was rapidly released in both circumstances. 4. In aqueous media the labile phosphate was released entirely as inorganic phosphate at faster rates with increasing alkalinity. 5. In acidified chloroform-alcohol mixtures the released phosphate appeared both as inorganic phosphate and different single (32)P-labelled organic phosphates, which were tentatively identified as the relevant mono-alkyl phosphates, presumably derived by acid-catalysed alcoholysis of a labelled microsomal component, or components. 6. The labile phosphate corresponded to the P exchangeable with non-radioactive ATP added during the enzyme reaction. 7. The possible molecular nature of the labile fraction of the bound phosphate is discussed.