Identification of Corticosteroid-Binding Globulin in Human Milk: Measurement with a Filter Disk Assay

Abstract

We have studied the concentration and properties of a protein which binds cortisol in human milk in samples obtained from women during the first 100 days after delivery. A filter disk assay was developed both for the measurement of plasma corticosteroid-binding globulin (CBG) and for the cortisol-binding protein in milk. The concentration of CBG in milk, expressed as its capacity to bind cortisol, is highest on the day of delivery, ca, 0.80 mug/dl, falls over the next 10 days to ca. 0.25 mug/dl, and remains at that level thereafter. If the concentration of CBG is expressed relative to the concentration of serum albumin in milk, it increases from day 1 to day 3 and then remains constant. A detailed comparison of CBG derived from milk and plasma showed that the two proteins co-migrated on Sephadex, sucrose gradient ultracentrifugation, and polyacrylamide gel electrophoresis. The two proteins had the same affinity for cotrisol, progesterone, 17-OH-progesterone, and dexamethasone. Furthermore, the binding activity of CBG in milk was neutralized with anti-CBG antibodies raised against CBG isolated from plasma. Unlike CBG, the concentration of cortisol in milk, 0.8-3.5 mug/dl, showed no systematic variation as a function of the postpartum day on which the sample was obtained.

Changes in the Concentration of Cortisol in Milk During Different Stages of Human Lactation

“…The eoncentration of glucocorticoids which were extracted from whole milk and measured by the CBG assay was slightly higher than the values reported by Rosner et al (1976), who used the whey fraction of milk for the CBG assay. This smalt difference is consistent with the finding in cows that oniy a small percentage (10% to 15%) of tnilk glucocorticoids occur in the lipid phase of milk (Tucker and Schwaltn, 1977),…”

“…The concentration of glucocorticoids in breast milk reported in earlier studies (Rapi, 1957;Ratsimamanga, Mouton and Bein, 1961) is much higher than that reported in a recent study (Rosner et at., 1976), probably because of the different extraction and assay methods used. Further studies of the concentration of glucocorticoids in breast milk are of interest beeause, first, the values in the mammary secretion may indicate the degree of exposure of the mammary epithelial cells to eirculating glucocorticoids, and, secondly, glucocorticoids ingested from breast milk may affeetthe growth and development of the suckling infant.…”

“…Cortisol also binds to albumin, which is present in milk (Rosner et al, 1976) and to the membrane of the milk fat globule (Tucker and Schwalm, 1977). Therefore, both CBG and afbumin may be involved in the transport of cortisol from blood into the mammary secretion.…”

“…1980) and in normal htiman mammary tissue in primary culture (Flaxman, 1974), Glucocorticoids are removed by the niatnmary gland from the peripheral circulation and pass into the mammary cells where they bind to eytoplasmic atid nuclear receptors (Tucker and Schwalm, 1977). A portion of glucocortieoids taken up by the mammary gland is excreted into milk (Koldovsky, 1980) where they bind to corticosteroid binding globulin (CBG), albumin and fat globules (Rosner^/a/., 1976;Payne er a/., 1976: Tucker andSchwalm, 1977).…”

Iodinated Compounds and Thyroxine Binding to Albumin in Human Breast Milk

“…T4 was not bound to a specific protein as in serum. This is different from the binding of milk cortisol (17). The amount of total proteins in mature milks was about '/6 of the serum protein contents and these results agreed with previous report (7).…”