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 x 10(10) M(-1)), specificity towards the lactogenic hormones and molecular weight (approximately 35 kDa), except for the more heterogeneous pig milk (approximately 43 to approximately 88 kDa) Furthermore, high affinity PRL-BP was also demonstrated in sheep milk, having pure lactogenic specificity and an Mr of approximately 35 kDa. Human milk contained a high affinity PRL-BP/GH-BP, which was recognized by both hPRL and hGH and also having an Mr of approximately 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 Mr of approximately 44 kDa (vs. approximately 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 degrees 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.
Amit
T
T
Department of Pharmacology, Bruce Rappaport Faculty of Medicine, Technion, Israel Institute of Technology, Haifa.
Dibner
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C
Barkey
R J
RJ
D000818:Animals; D002352:Carrier Proteins / Q000737:chemistry / Q000378:metabolism*; D002415:Cats; D004285:Dogs; D005260:Female; D013006:Growth Hormone / Q000378:metabolism*; D006736:Horses; D006358:Hot Temperature; D019382:Human Growth Hormone / Q000378:metabolism; D006801:Humans; D007700:Kinetics; D008892:Milk / Q000378:metabolism*; D008970:Molecular Weight; D011388:Prolactin / Q000378:metabolism*; D011817:Rabbits; D051381:Rats; D012756:Sheep; D013045:Species Specificity; D013552:Swine
D002352:Carrier Proteins; C029702:prolactin-binding protein; D019382:Human Growth Hormone; D011388:Prolactin; D013006:Growth Hormone; C051815:somatotropin-binding protein