**3. Lactose synthesis in the Golgi of MEC**

## **3.1 Lactose complex**

The first evidence of lactose synthesis in the Golgi of mammary alveolar cells dates back to 1980, when it was associated with the activity of galactosyltransferase and the presence of lactalbumin and bivalent metals such as manganese and calcium [27]. The lactose synthase (LS) synthetizes lactose (beta 1,4-galactoglucose) from UDP-galactose and glucose, and it is located specifically in trans-Golgi. The LS is an enzymatic complex of galactosyltransferase and LALB. LALB is only found in mammary epithelial cells, allowing galactosyltransferase to be specific for the formation of this disaccharide, making galactosyltransferase add galactose to glucose at even low concentration of glucose, increasing its affinity to this carbohydrate 1000-fold [23, 27]. In others cells, galactosyltransferase adds galactose to N-acetylglucosamine glycoconjugates, but in MEC, LALB changes substrate specificity from N-acetylglucosamine to glucose. In fact, the lactose synthesis depends directly on the amount of LALB associated with the galactosyltransferase that is inserted in the inner face of the Golgi apparatus membrane [28]. The LALB knockout produces a viscous, low-lactose milk difficult to remove from the mammary gland, highlighting the osmotic role of lactose in milk yield [29].

LALB expression increases immediately after delivery in pig and rodents and is regulated by lactogenic hormones [30–32]. LS has a Km of 1.5 mM for glucose and 60 μM for UDP-galactose; thus, the limiting stage in lactose synthesis is the availability of glucose in the Golgi [2, 23, 27]. Lactose synthesis begins in the first third of pregnancy but increases considerably after childbirth, as levels of placental sex steroids decrease, which has an inhibitory effect on lactose synthesis [1]. Lactose production remains relatively constant throughout the entire lactation process thanks to

**13**

*Lactose Synthesis*

*DOI: http://dx.doi.org/10.5772/intechopen.91399*

**3.2 Golgi's glucose transporters**

been found in late endosome and reticulum.

**3.3 Lactose synthesis regulation**

human, with 7.4% lactose, 2.0% protein, and 0.4% fat [9].

the action of prolactin and other lactogenic hormones and the stimulation associated with mammary gland emptying. The lactose is secreted in the milk together with LALB, α, β, and κ caseins, β-lactoglobulin, others nutrients, and immunomodulatory molecules [1]. Lactose represents around 5% of the milk content in all species, which revealed that is a highly conserved process. Also lactose is an osmotically active molecule, defining the water content in milk, which is in average 80% [1, 7]. In cows, in particular, milk is 5.0% lactose, 3.4% fat, and 2.3% protein. In seals, the lactose content of milk is minimal, and fat is predominant (50%), followed by 6.0% protein. This could be explained because pups need to double their weight in only 4 days to survive adverse environmental conditions [1]. On the other hand, human milk has a similar fat content to cow milk (3.7%), less protein content (1.0%), and more lactose (7.0 v/s. 5.0%). Donkeys presents similar content of macronutrients in milk to

The first studies related to glucose transport to the Golgi of MEC concluded that this was mediated by GLUT and SGLT transporters, since the transport of monosaccharides was inhibited by phloretin and phlorizin, known inhibitors of both types of transporters [33]. These vesicles present stereospecificity for several monosaccharides, such as D-glucose, L-glucose, D-xylose, 2-deoxy-D-glucose, and D-fructose. Moreover, the vesicles showed low permeability for glucosamine, a substrate of the GLUT1 transporter; for this reason, we assume that another glucose transporter is involved in the incorporation of glucose into this organelle. A decade later, another GLUT was identified in Golgi vesicles of MEC of late-lactating mice through Western blotting and binding studies of cytochalasin B, co-localizing with 110-kDa coatomer-associated protein β-COP [19, 24]. Interestingly, the results revealed that there is a second cytochalasin B-sensitive glucose transporter, which could correspond to GLUT8, cloned after such studies [34, 35]. In our last study, we were able to identify GLUT8 in the Golgi of lactating MEC in mice, co-localized with LALB, 58 K Golgi protein, and Golgi membrane-associated protein 130 [5]. Additionally, SGLT1 was identified in the Golgi of MEC from lactating cows, but no functional studies have been performed [23]. As SGLT is an active transporter that mobilize glucose thanks to electrochemical sodium gradient at the plasma membrane, more studies related to ion gradient between cytoplasm and inside the Golgi should be performed to really know the contribution of this transporter to glucose uptake into the Golgi of MEC. In **Figure 3**, we show glucose transporters present in the Golgi of mammary epithelial cell and their association with lactose synthesis. In summary, the reports highlight a variable increase in the expression of GLUT1, GLUT8, and GLUT12 in pregnancy and/or lactation in different models, including rodents and ruminants, but their responsibility in glucose uptake in the Golgi of mammary epithelial cells, an essential step to lactose synthesis, is not clear [8, 24, 36]. Moreover, although GLUT8 has been co-localized with Golgi proteins in MEC and in different compartments of endomembrane system in other cell types, GLUT1 has been found in the Golgi of MEC only in some of the species and strains studied, and its intracellular localization had been associated with mitochondria, which in not part of endomembrane system [5, 24, 33, 37]. In particular, GLUT8 has

The lactose synthesis depends principally on lactogenic hormones and glucose uptake in the Golgi of MEC. It starts in the first third of pregnancy, increasing

#### *Lactose Synthesis DOI: http://dx.doi.org/10.5772/intechopen.91399*

the action of prolactin and other lactogenic hormones and the stimulation associated with mammary gland emptying. The lactose is secreted in the milk together with LALB, α, β, and κ caseins, β-lactoglobulin, others nutrients, and immunomodulatory molecules [1]. Lactose represents around 5% of the milk content in all species, which revealed that is a highly conserved process. Also lactose is an osmotically active molecule, defining the water content in milk, which is in average 80% [1, 7]. In cows, in particular, milk is 5.0% lactose, 3.4% fat, and 2.3% protein. In seals, the lactose content of milk is minimal, and fat is predominant (50%), followed by 6.0% protein. This could be explained because pups need to double their weight in only 4 days to survive adverse environmental conditions [1]. On the other hand, human milk has a similar fat content to cow milk (3.7%), less protein content (1.0%), and more lactose (7.0 v/s. 5.0%). Donkeys presents similar content of macronutrients in milk to human, with 7.4% lactose, 2.0% protein, and 0.4% fat [9].
