On the isolation of a prolactin inhibiting factor (hormone)

A preparation of $\text{ca.}$ < 100 ng of a prolactin inhibiting factor was isolated which could be essentially pure, because of symmetrical single peaks by high pressure liquid chromatography. The $\text{in vitro}$ activity was at $\text{ca.}$ < 5 ng which is the highest potency reported by anyone. The paucity of $\text{ca.}$ < 100 ng/80,000 hypothalami necessitates patience for definitive data on more product from $\text{ca.}$ 240,000 to 450,000 hypothalami. Weight was estimated by comparing UV absorption at 220 nm with that of synthetic peptides. This preparation is not a catecholamine by chromatography, and gives new and timely credence to the concept that prolactin secretion is mediated by complex mechanisms including a peptide inhibiting factor and a catecholamine.