Characterization of a soluble prolactin-binding activity in rat liver cytosol

Soluble binding activity for lactogenic hormones has been detected in high-speed cytosolic preparations from the livers of 21-day-old male and female rats. No lactogenic hormone binding was detected in cytosols from rat heart, kidney, skeletal muscle or adipose tissue. Liver cytosol binding of 125I-labelled human growth hormone or ovine prolactin was dependent on time, temperature, and protein and calcium concentrations. Binding was specific for lactogenic hormones and not somatotrophic hormones. Scatchard analysis revealed linear plots with an affinity of 2.9-4.6 litres/nmol. By gel filtration the molecular weight of the lactogen-binding activity was greater than 450,000. The cytosolic binding activity may be an internalized form of the membrane-bound lactogen receptor, which has similar binding characteristics, or alternatively it may be a distinct binding species, with a defined role in mediating intracellular effects of prolactin in the liver.