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Milk Composition & Synthesis
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Milk Synthesis

Lactose Synthesis

Lactose is a disaccharide composed of galactose and glucose covalently bound by a 1-4 glycosidic linkage. Lactose is also known as 4-O--D-galactosylpyranosyl-a-D-glucopyranoside. Lactose is only found in milk Lactose is the primary carbohydrate in milk of most species.

Molecular structure of lactose.

The lactose synthesis pathway is shown in the figure below. The following points are relevant to this figure (as indicated by the numbers on the figure; see below the figure for the legend of abbreviations):

  1. One glucose is converted to UDP-glucose, which in turn is converted to one UDP-galactose. Another glucose is used for lactose synthesis without modification. Therefore, 2 glucoses are required for each lactose molecule synthesized.
  2. Glucose passes across the Golgi membrane into the Golgi lumen by a glucose transporter (GLUT 1). The presence of GLUT 1 on the Golgi membrane apparently is specific to the mammary epithelial cell, as most cells do not have this glucose transporter on the Golgi membrane. The transport of glucose is not active (not requiring energy), and is apparently not rate limiting. But it is affected by glucose levels in the cytoplasm. The Golgi is shown in the image below, along with a secretory vesicle (SV) that contains a casein micelle (arrow).
  3. UDP-galactose is actively transported into the Golgi lumen, and transport of UDP-galactose into the Golgi lumen may be rate limiting to lactose synthesis. UDP-glucose is not transported into the Golgi.
  4. Lactose is a nonpermeable disaccharide which can not diffuse out of the Golgi membrane or out of secretory vesicles' membrane. This characteristic is important for milk synthesis because it is the synthesis of the nondiffusible lactose which results in water being drawn into the Golgi.
  5. The UDP generated from lactose synthesis could be inhibitory to lactose synthesis if it accumulated in the Golgi lumen. However, UDP is rapidly hydrolyzed into UMP and inorganic P by nucleoside diphosphatase (NDPase). UMP is actively removed from the Golgi, while the inorganic P diffuses out of the Golgi.
  6. The lactose synthesis reaction is essentially one-way, that is, lactose is not hydrolyzed to form glucose and galactose. The very high levels of lactose do not inhibit its own synthesis.
  7. The lactose synthase enzyme activity is composed of:
      1. an "A" protein = galactosyltransferase (GT)
      2. a "B" protein = a-lactalbumin (a-LA)

Abbreviations: GT = galactosyltransferase; a-LA = a-lactalbumin; NDPase = nucleotide diphosphatase; Pi = inorganic phosphate; PPi = inorganic diphosphate; UDP = uridine diphosphate; UDP-galactose = Uridine diphosphate galactose; UDP-glucose = uridine diphosphate glucose; UMP = uridine monophosphate; UTP = uridine triphosphate.

Galactosyltransferase (GT)

ß1,4 Galactosyltransferase (GT) is the enzymatic subunit of lactose synthase. It is a glycoprotein with a molecular weight varying from 35-60 kDa, depending upon the amount of glycosylation and the degree of proteolytic degradation. ß1,4 Galactosyltransferase in milk is proteolytically clipped removing the cytoplasmic and transmembrane domains. The GT found in milk has a molecular weight of 35-45 kDa. Without the presence of a-lactalbumin, the enzyme functions in the Golgi during glycoprotein biosynthesis to add galactose to oligosaccharides with terminal N-acetylglucosamine residues in a ß1-->4 linkage. The GT transfers galactose from the donor, UDP-galactose, to the terminal N-acetylglucosamine (GlcNAc) acceptor on the oligosaccharise complex of glycosylated proteins. Galactosyltransferase is found in most tissues of the body. It is only found on the inner surface of the Golgi apparatus.

ß1,4 Galactosyltransferase is unique among all glycosyltransferases in that its substrate specificity can be modified by addition of a-lactalbumin. Together, ß1,4 galactosyltransferase and a-lactalbumin form the lactose synthase complex. Because a-lactalbumin is only expressed in the mammary gland, lactose synthesis only occurs in the mammary gland. In addition, expression of the a-lactalbumin gene is closely regulated by hormones, so that lactose synthesis only occurs during the lactating state of the tissue.

a-Lactalbumin (a-LA)

  • Is a milk whey protein and accounts for 25% of the total whey proteins in cow milk, and accounts for 2 to 5% of total skim milk proteins.
  • It is not catalytically active by itself, but is necessary for the synthesis of lactose.
  • Has similarities in structure to lysozyme. Lysozyme is a glycosidase which hydrolyses of the polysaccharides in bacterial cell walls.
  • a-LA alters the sugar-binding properties of galactosyltransferase so that lactose is produced rather than the usual function of galactosyltransferase.

As secretory vesicles fuse with the apical surface of the mammary cell they dump the soluble a-LA into the lumen, but much of the galactosyltransferase stays bound to the apical membrane. Galactosyltransferase activity can be found on milk fat membranes (remember, the milk fat membrane is derived from the mammary cell apical membrane). Some GT can be found in milk, too (see above).

Bovine, murine and porcine a-lactalbumin genes have all been sequenced and their proteins have molecular weights of about 14 kD. a-Lactalbumin is produced at a concentration of approximately 0.2 to 1.8 mg/ml in the milk of most mammals. It is synthesized in the rough endoplasmic reticulum and passes to the Golgi complex where it interacts with GT. In the mammary Golgi apparatus, a-LA combines with GT and alters the substrate specificity of galactosyltransferase from N-acetylglucosamine (GlcNAc) to glucose. This modified complex transfers galactose to glucose rather than to N-acetylglucosamine.

Lactose cannot diffuse out of the Golgi and secretory vesicles, so water is drawn into the vesicles to balance the osmotic pressure. Since lactose synthase is necessary for production of lactose and the subsequent movement of water into the mammary secretory vesicles, it is critical in the lactational control and secretion of milk. The substrates for lactose synthase do not appear to be limiting since results of many experiments indicate that infusion of glucose, increasing blood glucose, does not increase milk production. Addition of more than 0.5 mg/ml glucose (low levels) to dispersed mammary cells in vitro also does not increase milk production. There is a suggestion that glucose transport across the plasma membrane may be limiting, but this is inconclusive. The expression of the glucose transporter is not up-regulated in cattle when exogenous growth hormone is administered. This suggests that glucose transport across the plasma membrane is not normally a limiting factor in milk production. The glucose transporter from the cell cytoplasm to the Golgi has a very large capacity and it seems unlikely that glucose transport across the Golgi membrane is limiting.

In vitro studies suggest that the rate of lactose synthesis appears to be dependent upon the a-LA to GT ratio. The maximal lactose synthesis activity was observed at an a-LA to GT molar ratio of 500 to 1. Molar ratios of a-LA to GT greater than 500 to 1 did not increase lactose synthesis in this in vitro system. The actual molar ratio of a-LA to GT in the Golgi of the mammary gland is not known.

Lactose synthesis is controlled by hormones involved in lactogenesis (see the Lesson on Mother and Neonate: Lactogenesis).

Effects of Underfeeding on Milk Composition

Underfeeding cows reduces milk production and lactose percentage, but increases fat percent. Subsequent feeding with an adequate ration reverses these symptoms. As a general rule, any ration that increases milk production usually reduces the fat percentage of milk.

Lactose is relatively insensitive to changes in the cow's diet. About the only nutritional regimen that alters lactose is underfeeding, in which case lactose percentage is slightly reduced.

Milk Composition & Synthesis