Characterization of prolactin- and growth hormone-binding proteins in milk and their diversity among species

The present study was undertaken to identify and characterize the diversity and species distribution of soluble prolactin binding-protein (PRL-BP) and growth hormone-binding protein (PRL-BP) in mammalian milk. We previously divided mammalian serum GH-BP into four main groups and identified a GH-BP with shared lactogenic/somatogenic properties in rabbit, horse, dog, pig and cat (Type III species). Here we describe PRL-BP in milk of Type III species and show it is relatively conserved within the group, having similar characteristics in terms of binding affinity for hGH (0.74−5.5×10 10 M −1), specificity towards the lactogenic hormones and molecular weight (∼35 kDa), except for the more heterogeneous pig milk (∼43 to ∼88 kDa) Furthermore, high affinity PRL-BP was also demonstrated in sheep milk, having pure lactogenic specificity and an M r of ∼35 kDa. Human milk contained a high affinity PRL-BP/GH-BP, which was recognized by both hPRL and hGH and also having an M r of ∼35 kDa. In rabbit milk a separate GH-BP was also detected; it was clearly distinguished from the corresponding milk PRL-BP on the basis of its M r of ∼44 kDa (vs. ∼32 kDa for PRL-BP), its shared lactogenic/somatogenic hormonal specificity (vs. purely lactogenic for PRL-BP) and also on the basis of its relative resistance to heating at 56°C for up to 3 h, while PRL-BP activity was completely destroyed within 30 min. This diversity of milk PRL-BP and GH-BP among mammalian species fits in with our earlier classification of serum GH-BP and also with the reported evolutionary rates of PRL and GH; this suggests these BPs may play important species-specific roles in the suckling newborn and/or maternal mammary gland, in keeping with the functions described for GH-BP.