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The nature of the proteins released together with progesterone from the bovine corpus luteum was investigated. Slices of bovine corpus luteum obtained at the mid-luteal stage of the oestrous cycle were incubated in vitro in the presence of bovine LH. Radioactive monitoring of compounds separated by gas–liquid chromatography showed that added [¹⁴C]acetate was converted to sterols and progesterone within the 2-h incubation period. Electrophoresis of the proteins recovered from the incubation medium showed the presence of a major band which migrated at the same rate as a progesterone-binding protein isolated from pooled corpora lutea. Densitometry revealed that the proteins in the incubation medium were enriched threefold in binding protein compared to those in an homogenate of corpus luteum. Equilibrium dialysis of the medium after addition of [³H]progesterone revealed the presence of a progesterone-binding activity (association constant = 10⁶l/mol) analogous to that contained in the cytosol fraction from bovine corpus luteum.

The finding that progesterone-binding protein is released concomitantly with the hormone has implications for the granule hypothesis of steroid secretion. The existence of an intracellular steroid-binding protein complex to maintain progesterone in high intragranular concentration would be predicted from this hypothesis.