The nature of absorbance changes following the addition of ADP to mitochondria: ATP synthesis from intramitochondrial inorganic phosphate.

The transient absorbance increase induced by ADP in phosphate-loaded respiring mitochondria becomes stable and greatly amplified by inhibitors of phosphate transport. The absorbance changes are sensitive to oligomycin and to aurovertin and their extent is proportional to the amount of ADP added. Simultaneously with the ADP-dependent increase in absorbance the inorganic phosphate and K(+)-ion content of the matrix decreases. It is concluded that the optical change reflects contraction of the matrix compartment secondary to intramitochondrial solute changes.