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

The research article investigates the diversity of soluble prolactin binding-protein (PRL-BP) and growth hormone-binding protein (GH-BP) in the milk of different mammalian species. The study further attempts to characterize these proteins and explore their potential implications.

Characterization of Prolactin- and Growth Hormone-Binding Proteins

• The study delved into the characterization of soluble prolactin-binding protein (PRL-BP) and growth hormone-binding protein (GH-BP) in mammalian milk. This built on the researchers’ previous work where they divided mammalian serum GH-BP into four main groups.
• The researchers found that PRL-BP in milk from certain species – namely rabbits, horse, dog, pig, and cat (Type III species), exhibited relative conservation and demonstrated similar binding affinity for human growth hormone (hGH). However, pig milk displayed greater heterogeneity.
• Hormonal specificity towards lactogenic hormones (involved in lactation) and similar molecular weight across these Type III species was also observed.

Purity of Lactogenic Specificity and Presence of PRL-BP/GH-BP

• This research also highlighted that high affinity PRL-BP was identified in sheep milk, showing pure lactogenic specificity and a similar molecular weight to the PRL-BP in Type III species.
• Human milk showed the presence of a high-affinity PRL-BP/GH-BP that was recognized by both human prolactin (hPRL) and hGH and held an approximately similar molecular weight as that of sheep PRL-BP and Type III PRL-BP.

Diversity of PRL-BP and GH-BP among Mammalian Species

• The study detected a separate and distinct GH-BP in rabbit milk, based on its molecular weight, its shared lactogenic/somatogenic hormonal specificity, and also on its relative resistance to heating up to 56 degrees Celsius for up to 3 hours. This was in contrast to PRL-BP activity, which was completely destroyed within 30 minutes at the same temperature.
• The findings underline the diversity of PRL-BP and GH-BP among different mammalian species, which aligns with the earlier classification of serum GH-BP by the same researchers, and the reported evolutionary rates of prolactin and growth hormone.
• The conclusion drawn from this study suggested these binding proteins may play species-specific roles in the suckling newborn and/or maternal mammary gland, echoing the functions described for GH-BP.