Measurement ofβ‐casein in human milk by microparticle‐enhanced nephelometric immunoassay

Measurement of nine human milk proteins by nephelometric immunoassays: application to the determination of mature milk protein profile

OBJECTIVES

Microparticle-enhanced nephelometric immunoassays for six human milk proteins (beta-casein, kappa-casein, alpha-lactalbumin, serum albumin, lactoferrin, and lysozyme) and conventional immunonephelometry assays for immunoglobulin A, C3, and C4 complement proteins were developed and characterized.

DESIGN AND METHODS

Microparticle-enhanced nephelometric immunoassays are competitive assays based on the nephelometric quantification of the inhibition of microparticle-protein conjugates immunoagglutination by the proteins to be assayed.

RESULTS

High precision (CVs ranged from 1% to 14% in within- and between-assays) and recovery (linear recovery in dilution-overloading assay) ensure a reliable determination of the main human milk proteins by single-step homogeneous nephelometric immunoassays, accurate over wide ranges of concentration. These immunoassays were easily applied to a large number of mature human milk samples (between 373 and 503 according to the proteins tested).

CONCLUSIONS

The immunoassays developed could be applied to the fast determination of human milk protein profile usable for nursery milk bank and fortification.

Immunological and nutritional composition of human milk in relation to prematurity and mother's parity during the first 2 weeks of lactation

BACKGROUND

To investigate the effect of prematurity and parity on the dynamics of the major immunologic and nutritional proteins of human milk over the first 2 weeks of lactation.

METHODS

Microparticle-enhanced nephelometric immunoassays were developed for the quantification of alpha-lactalbumin, beta-casein, serum albumin, lactoferrin, and lysozyme in human milk. These components, immunoglobulin A, and total proteins were assayed in 368 individual samples collected from 74 mothers.

RESULTS

The dynamics of the major immunologic and nutritional proteins in early lactation presented similar patterns in preterm and term human milks. In comparison with term milk, preterm milk was globally characterized by higher concentrations of immune proteins and lower concentrations of nutritive proteins. These differences were increased by the degree of prematurity, which, however, influenced the absolute and relative protein concentrations differently, depending on the stage of lactation. The protein composition of term milk was similar, whatever the mother's parity. Conversely, the influence of prematurity on the levels of milk proteins during the first days of lactation was even greater in primiparous mothers.

CONCLUSIONS

This precise description of the composition of preterm and term milk, regarding the main nutritional and immunologic proteins, confirms the influence of both prematurity and parity on milk components and demonstrates the combined effect of these two conditions.

Microparticle-enhanced nephelometric immunoassay of lysozyme in milk and other human body fluids

Quantitation of lysozyme in human milk was performed by a microparticle-enhanced nephelometric immunoassay based on the measurement of the light scattered during the competitive immunoagglutination of a microparticle–lysozyme conjugate with an anti-lysozyme antiserum. This immunoassay has a detection limit of 8 μg/L of reaction mixture and can be performed using diluted milk (1:6000, in reaction mixture), excluding sample pretreatment. Human milk lysozyme can be quantified over the concentration range 0.09–1.50 g/L, with within- and between-run coefficients of variation <5%. Changes in the lysozyme concentration of human milk during lactation were determined in 636 samples. Lysozyme concentrations (mean ± SE) decreased from colostrum (0.36 ± 0.02 g/L) to transitional milk (0.30 ± 0.01 g/L) and mature milk during days 15–42 (0.30 ± 0.01 g/L), then increased in the mature milk during days 43–56 (0.35 ± 0.01 g/L) and especially during days 57–84 (0.83 ± 0.05 g/L). The proportion of lysozyme contributing to total protein was found to rise during lactation and was as follows: colostrum (1.7%), transitional milk (2.3%), and mature milk from days 15–28 (2.7%), days 29–42 (3.1%), days 43–56 (3.8%), and days 57–84 (7.3%). The assay developed for milk was also suitable for the determination of lysozyme in other human body fluids.