Isolation of two different pools of pituitary prolactin.

Two pools of prolactin (Prl) were isolated from the pituitary gland. One readily soluble after homogenization and the other sedimented with secretory granules. In ovariectomized-oestrogen treated rats the latter represents the major Prl fraction of pituitary and can be recovered as immunoreactive Prl after extraction in 2.5 M urea. By correlation of electron microscopy, chromatography and RIA, evidence is gathered that the soluble fraction is constituted by monomeric (little) Prl and that secretory granules contain Prl in a polymerized form (big).