Insulin-Like Growth Factors I and II and Their Binding Proteins in Human Milk

Abstract

Insulin-like growth factors (IGF-I and -II) are peptide growth factors that contribute to the growth-promoting properties of human milk. IGFs in extra cellular fluids are associated with high-affinity binding proteins (IGFBPs). In this study, IGF-I and -II, and IGFBPs in human milk were characterized, and the stability of the IGFs and IGFBPs to heat treatment was investigated. IGF-I and -II were quantified by radioimmunoassays of acid-chromatographed samples, and IGFBPs were characterized using Western ligand blotting. The concentration (mean +/- SD) of IGF-I in human milk was 1.5 +/- 0.5 micrograms/L, compared to 2.7 +/- 0.7 micrograms/L for IGF-II. Heat treatment did not significantly affect either IGF-I or -II content. Human milk contains a single, nonglycosylated, IGFBP, with an apparent Mr of 31 k, which was immunoprecititable by an antibody to IGFBP-2. Stability of the IGFBP to heat treatment was assessed and was not significantly affected by heat treatment. Therefore, both IGF-I and -II, and the IGFBP in human milk appear to be stable under heat treatment conditions routinely used for processing banked human milk.