Inorganic phosphate is the major component of the thermostable cytoplasmic fraction which stimulates mitochondrial anion uniport.

A low molecular weight thermostable cytoplasmic fraction isolated from rat liver homogenate when pre-incubated with mitochondria increases the rate at which anions enter mitochondria via the pH-dependent anion-conducting channel in the inner membrane. The crude fraction obtained by centrifuging and heating the liver homogenate was purified by gel filtration and chromatography on DEAE-cellulose. The resulting factor is stable to heating at 100 degrees C, freeze-drying and extremes of pH. Inorganic phosphate co-purified with activity and activity was lost when the phosphate was removed by barium salt precipitation. A pure sample of KH2PO4 produced stimulation of anion conductivity. These results show that the major portion of the activity which stimulates anion uniport can be accounted for by the presence of phosphate in the crude and purified fractions. Mersalyl blocks stimulation when added before, but not when added after, incubation with phosphate which shows that the stimulation is produced by phosphate in the mitochondrial matrix. The proposed role of this factor in thyroid hormone action is discussed in the light of its identification as inorganic phosphate.