Changes in protein and nutrient composition of milk throughout lactation in dogs.

OBJECTIVE: To evaluate changes in protein and nutrient composition of milk throughout lactation in dogs. SAMPLE POPULATION: Milk samples collected from 10 lactating Beagles. PROCEDURE: Milk samples were collected on days 1, 3, 7, 14, 21, 28, 35, and 42 after parturition and analyzed to determine concentrations of nitrogen, nonprotein nitrogen, casein, whey proteins, amino acids, lipids, lactose, citrate, minerals, and trace elements. Optimum conditions for separating casein from whey proteins and distribution of milk proteins throughout lactation were assessed by use of polyacrylamide gel electrophoresis. RESULTS: Protein concentration was high in samples collected on day 1 (143 g/L), decreased through day 21 (68.4 g/L), and increased thereafter. Concentration of nonprotein nitrogen did not change throughout lactation (5.7 to 9.9% of total nitrogen content). Casein-to-whey ratio was approximately 70:30 and remained constant throughout lactation. Lactose concentration increased from 16.6 g/L on day 1 to 34.0 to 40.2 g/L on days 7 to 42. Lipid concentration ranged from 112.5 to 1372 g/L. Citrate concentration increased from day 1 (4.8 mM) to day 7 (6.6 mM), then gradually decreased until day 42 (3.9 mM). Iron, zinc, copper, and magnesium concentrations decreased during lactation, whereas calcium and phosphorus concentrations increased. Calcium-to-phosphorus ratio remained constant throughout lactation (approx 1.6:1). Energy content of milk ranged from 1,444 to 1,831 kcal/L. CONCLUSIONS AND CLINICAL RELEVANCE: Protein and nutrient composition of milk changes throughout lactation in dogs. These data can provide valuable information for use in establishing nutrient requirements of puppies during the suckling period.

High affinity binding of the transcobalamin II-cobalamin complex and mRNA expression of haptocorrin by human mammary epithelial cells.

Little is known about the acquisition of cobalamin by the mammary gland and its secretion into milk. Human milk and plasma contain at least two types of cobalamin binding proteins: transcobalamin II (TC) and haptocorrin (HC). In plasma, TC is responsible for the transport of cobalamin to tissues and cells; however, cobalamin in milk is present exclusively bound to HC. We show that human mammary epithelial cells (HMEC) exhibit high affinity for TC; Scatchard analysis revealed a single class of binding sites for the TC-[(57)Co]cyanocobalamin complex with a dissociation constant (K(d)) of 4.9 x 10(-11) M. Uptake of the TC-[(57)Co]cyanocobalamin complex at 37 degrees C was saturable by 24 h. Binding of free [(57)Co]cyanocobalamin to HMEC was not saturable and very limited binding of the HC-[(57)Co]cyanocobalamin complex was observed. Expression of the haptocorrin gene by HMEC was confirmed by Northern blot and PCR analysis. Thus, a specific cell surface receptor for the TC-cobalamin complex exists in the mammary gland and once cobalamin is internalized, it may be transferred to HC and subsequently secreted into milk as a HC-cobalamin complex.

Binding of transcobalamin II by human mammary epithelial cells.

The presence of nutrient binders in milk may have an important role during milk production and may influence the nutrient's bioavailability to the infant. Human milk and plasma contain at least two types of vitamin B12 binders: transcobalamin II (TCII) and haptocorrin (Hc). Vitamin B12 in milk is exclusively bound to Hc (Hc-B12). In plasma, the major vitamin B12 binding protein that is responsible for delivering absorbed vitamin B12 to most tissues and cells is TCII (TCII-B12). Currently, little is known about the route of secretion of vitamin B12 into human milk. It is possible that a receptor-mediated pathway is involved, since maternal vitamin B12 supplementation increases the amount of the vitamin secreted into human milk if the mother's vitamin B12 consumption is low, but remains unchanged if her intake is adequate. In this study, we investigated the process by which the mammary gland acquires vitamin B12 from maternal circulation, whether as a free vitamin or as a Hc-B12 or TCII-B12 complex. TCII was purified from plasma incubated with [57Co]vit B12 (B12*), while Hc was purified from whey incubated with B12*. Both proteins were separated by fast protein liquid chromatography using gel filtration and anion-exchange columns. Purity of the separated proteins was assessed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Binding studies were carried out on a monolayer of normal human mammary epithelial cells (HMEC) at 4 degrees C using free B12* and TCII-B12* and Hc-B12* complexes. Minimal binding of free B12* and Hc-B12* to HMEC was observed; however, HMEC exhibited a high affinity for the TCII-B12* complex. This study suggests that a specific cell surface receptor for the TCII-B12 complex exists in the mammary gland. It is possible that once vitamin B12 is in the mammary gland it is transferred to Hc (which may be synthesized by the mammary gland) and then secreted into milk as a Hc-B12 complex.

Changes in nutrient and protein composition of cat milk during lactation.

OBJECTIVE: To evaluate changes in the nutrient and protein composition of cat milk during lactation. ANIMALS: 12 lactating domestic shorthair cats. PROCEDURE: Milk samples collected on days 1, 3, 7, 14, 28, and 42 after parturition were analyzed for concentrations of nitrogen, nonprotein nitrogen, casein, whey proteins, amino acids, total lipids, lactose, citrate, minerals, and trace elements. Individual milk proteins (caseins and whey proteins) were analyzed by use of polyacrylamide gradient gel electrophoresis. RESULTS: True protein concentration ranged from 6.3 to 8.6% and was as high in mature milk as in colostrum. Nonprotein nitrogen as a portion of total N was constant (approx 8%), as was the whey-to-casein ratio (approx 50:50). Total lipid concentration was high (9.3%) in colostrum, rapidly decreased, then increased to 9% in mature milk. Lactose concentration was constant at 4%. Milk calcium, iron, and copper concentrations increased markedly during lactation, and magnesium and zinc values remained constant. Colostrum and early milk had a low Ca-to-P ratio of 0.4:0.9. Although calcium concentration increased with time, phosphate concentration also increased so that the Ca-to-P ratio remained constant in mature milk at 1.0: 1.2. The major whey proteins had molecular weights of approximately 14,000, 19,000, 40,000 and 80,000. The 80,000 protein (possibly lactoferrin) decreased in concentration during lactation. Two major casein subunits of approximately 28,000 and 33,000 were found, and both increased during early lactation. CONCLUSIONS: Nutrient composition of cat milk and, thus, provision of nutrients to nursing kittens changes over time.