Bioactive and immunoreactive prolactin variants in human milk

Area of Science:

• Endocrinology
• Lactation Biology
• Neonatal Nutrition

Background:

• Prolactin (PRL) plays a crucial role in lactation and infant development.
• Previous studies may have underestimated PRL's bioactivity in human milk due to measurement methods.

Purpose of the Study:

• To quantify and characterize prolactin-like bioactivity (B) and immunoreactivity (I) in human milk from mothers of term and preterm infants across different lactation stages.
• To compare milk PRL bioactivity with serum PRL levels.
• To investigate the forms and modifications of PRL in human milk.

Main Methods:

• Collected human milk from term (T) and preterm (PT) mothers at early, mature, and late lactation stages.
• Measured PRL-like bioactivity using Nb2 cell proliferation assay.
• Determined PRL immunoreactivity using radioimmunoassay (RIA).
• Analyzed PRL molecular mass, phosphorylation, and glycosylation.

Main Results:

• Early lactation milk showed significantly higher PRL-like bioactivity (B) compared to immunoreactivity (I), particularly in preterm infants (PT:B/I ratio = 3.6).
• PRL-like B and I levels decreased as lactation progressed.
• Bioactive PRL in milk was predominantly found in phosphorylated forms and existed in multiple molecular weight variants.
• Treatment of glycosylated milk fractions increased bioactivity, suggesting a role for glycosylation.

Conclusions:

• Human milk possesses considerably greater PRL-like bioactivity than previously estimated by RIA alone.
• The presence of diverse bioactive PRL variants in milk suggests a regulatory mechanism influencing neonatal development.
• Further research into milk PRL's role in infant health is warranted.