Journal of Dairy Research (1992) 59 161-107
Printed in Great Britain
161
Calcium, phosphate and citrate in human milk at initiation of lactation BY JACQUELINE C. KENT, PETER G. ARTHUR, ROBERT W. RETALLACK* AND PETER E. HARTMANN Department of Biochemistry, The University of Western Australia, Nedlands and * Department of Endocrinology and Diabetes, QE11 Medical Centre, Nedlands, WA 6009, Australia {Received 3 June 1991 and accepted for publication 30 September 1991) The onset of copious milk secretion (laetogenesis II) in women occurs between 1 and 3 d after birth, and during this period the composition of breast milk changes. During the first 5 d of lactation we measured the concentrations of total, diffusible and ionized Ca (Catot, Cad, Ca2+), diffusible phosphate (Pid), diffusible citrate (Citd) and lactose in the breast milk. On day 1 after birth the concentrations (mean + SEM) were Catot, 5-71±O30 HIM; Cad, 2-66±019 mat; Ca2+, 2-90 + 0-18 mat; Pi d , 0-26±0-16 mai; Citd, 0-25 + 0-03 HIM and lactose, 76+11 mat. Between day 1 and day 4 the concentration of Catot increased 1-7-fold to 9-56 + 0-39 mM, Cad increased 1-8-fold to 4-75±0-26 mM, Ca2+ decreased by 20% to 2-33 + 0-13 mM, Pi d increased 6-6-fold to 1-69 + 0-11 mM, Citd increased 20-fold to 5-06 + 0-21 mM, and lactose increased 2-3-fold to 173±4mM. A high correlation has been found between [Cad] and [Citd] in the milk of both ruminant and non-ruminant species, which show a wide range in concentrations of [Cad] and [Citd], and the data fit a simple physicochemical model of ion equilibria in the aqueous phase of milk. The results of the present study confirm the relationship between [Cad] and [Citd] in human milk, even during laetogenesis II when the composition of the milk is changing very rapidly. SUMMARY.
Ca, phosphate, citrate and lactose are secreted into milk by the Golgi vesicle (Kuhn & White, 1975; Linzell el al. 1976; Neville & Peaker, 1979). Ca is actively transported into the Golgi vesicle from the cytosol (Holt, 1981) and occurs in milk in two forms: bound to casein, or in a diffusible state. Diffusible Ca (Cad) comprises ionized Ca (Ca2+) and Ca complexed with inorganic anions such as citrate and phosphate. Lactose is synthesized in the Golgi vesicle from glucose and uridine 5'diphosphogalactose (UDPgalactose). The UDP released is cleaved to uridine 5'monophosphate and orthophosphate which equilibrates with the cytosol (Holt, 1981). Citrate diffuses across the Golgi vesicle membrane from the cytosol (Holt, 1981). Holt & Muir (1979) have proposed a model to explain the interrelationships between the concentrations of Cad, phosphate and citrate in milk: [Cad] = [Ca»]+ , , \:
Printed in Great Britain
161
Calcium, phosphate and citrate in human milk at initiation of lactation BY JACQUELINE C. KENT, PETER G. ARTHUR, ROBERT W. RETALLACK* AND PETER E. HARTMANN Department of Biochemistry, The University of Western Australia, Nedlands and * Department of Endocrinology and Diabetes, QE11 Medical Centre, Nedlands, WA 6009, Australia {Received 3 June 1991 and accepted for publication 30 September 1991) The onset of copious milk secretion (laetogenesis II) in women occurs between 1 and 3 d after birth, and during this period the composition of breast milk changes. During the first 5 d of lactation we measured the concentrations of total, diffusible and ionized Ca (Catot, Cad, Ca2+), diffusible phosphate (Pid), diffusible citrate (Citd) and lactose in the breast milk. On day 1 after birth the concentrations (mean + SEM) were Catot, 5-71±O30 HIM; Cad, 2-66±019 mat; Ca2+, 2-90 + 0-18 mat; Pi d , 0-26±0-16 mai; Citd, 0-25 + 0-03 HIM and lactose, 76+11 mat. Between day 1 and day 4 the concentration of Catot increased 1-7-fold to 9-56 + 0-39 mM, Cad increased 1-8-fold to 4-75±0-26 mM, Ca2+ decreased by 20% to 2-33 + 0-13 mM, Pi d increased 6-6-fold to 1-69 + 0-11 mM, Citd increased 20-fold to 5-06 + 0-21 mM, and lactose increased 2-3-fold to 173±4mM. A high correlation has been found between [Cad] and [Citd] in the milk of both ruminant and non-ruminant species, which show a wide range in concentrations of [Cad] and [Citd], and the data fit a simple physicochemical model of ion equilibria in the aqueous phase of milk. The results of the present study confirm the relationship between [Cad] and [Citd] in human milk, even during laetogenesis II when the composition of the milk is changing very rapidly. SUMMARY.
Ca, phosphate, citrate and lactose are secreted into milk by the Golgi vesicle (Kuhn & White, 1975; Linzell el al. 1976; Neville & Peaker, 1979). Ca is actively transported into the Golgi vesicle from the cytosol (Holt, 1981) and occurs in milk in two forms: bound to casein, or in a diffusible state. Diffusible Ca (Cad) comprises ionized Ca (Ca2+) and Ca complexed with inorganic anions such as citrate and phosphate. Lactose is synthesized in the Golgi vesicle from glucose and uridine 5'diphosphogalactose (UDPgalactose). The UDP released is cleaved to uridine 5'monophosphate and orthophosphate which equilibrates with the cytosol (Holt, 1981). Citrate diffuses across the Golgi vesicle membrane from the cytosol (Holt, 1981). Holt & Muir (1979) have proposed a model to explain the interrelationships between the concentrations of Cad, phosphate and citrate in milk: [Cad] = [Ca»]+ , , \:
