PROLACTIN BINDING IN HUMAN-MILK AT DIFFERENT STAGES OF GESTATION AND DURATION OF LACTATION IN RELATION TO NUTRIENT COMPOSITION

A prolactin binding protein was isolated and characterized in microsom es isolated from milk collected from mothers who delivered infants pre maturely (28-34 wk) and at term. Scatchard analysis of (I-125) human p rolactin binding to milk microsomal membranes revealed a single class of specific binding sites having a mean K(D) and binding capacity of 4 .2 +/- 0.4 nM and 116.9 +/- 12.4 fmol/mg membrane protein, respectivel y. Comparison between prolactin binding protein content and nutrient c omposition of milk at different stages of gestation and lactation was made. Total prolactin receptor content in milk increased with duration of lactation (0.23 +/- 0.03, 0.53 +/- 0.3, 0.55 +/- 0.10 fmol/mL on d ays 4,16, and 37 postpartum, respectively). No apparent effect of gest ation on receptor content and binding affinity was observed. A negativ e correlation between concentration of prolactin and the lipid and lac tose content of milk was observed.