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 microsomes isolated from milk collected from mothers who delivered infants prematurely (28-34 wk) and at term. Scatchard analysis of (I-125) human prolactin 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, respectively. Comparison between prolactin binding protein content and nutrient composition 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 days 4,16, and 37 postpartum, respectively). No apparent effect of gestation on receptor content and binding affinity was observed. A negative correlation between concentration of prolactin and the lipid and lactose content of milk was observed.