Lactose Properties and Synthesis

**3**

**Chapter 1**

*Néstor Gutiérrez-Méndez*

**1. The biological role of lactose**

nourishment of newborn mammals [2, 3].

their transport into the enterocytes through the Na+

even the death [3].

Introductory Chapter: Lactose

Milk provides infants with essential nutrients to support the first months of life. Newborns and young animals obtain their energy mostly from milk lipids and lactose (∼17 kJ per gram of lactose). Only lactose provides 40% of the energy needs of suckling mammals. This fact explains why almost all the mammalian milk contains 40-75 g of lactose per litter, and why the milk of mammals is the only source in nature with a significant content of lactose [1–3]. Congenital deficiency to digest lactose is rare in baby mammals since it can lead to growth delay, dehydration, and

Lactose is a disaccharide synthesized in the mammary gland of mammals, and only scarce plant species show this saccharide. The Golgi vesicles of mammary epithelial cells synthesize lactose from two molecules of glucose. One of this glucose is first epimerized to galactose (Leloir pathway) and phosphorylated. Then, condensation with the other glucose occurs through the lactose synthetase system. This system comprises the enzyme galactosyl transferase and the protein modifier α-lactalbumin. When the protein modifier binds to the galactosyl transferase, it catalyzes the synthesis of lactose from uridine-diphosphate-galactose (UDP-gal) and glucose [1–3]. In the absence of the protein modifier, the galactosyl transferase does not synthesize lactose and instead catalyzes the synthesis of N-acetyl lactosamine on glycoproteins. This last reaction occurs in most tissues, but in the mammary gland of women after giving birth, the increase in prolactin and a decrease in progesterone hormones induce the formation of the protein modifier (α-lactalbumin). Consequently, the breast can synthesize lactose in the milk for the

Lactose digestion in humans involves the action of intestinal lactase. Lactose is a disaccharide containing galactose and glucose linked by a β 1-4 glycosidic bond. This sugar cannot be transported across the epithelial cell membrane into the enterocytes and then into the bloodstream as a disaccharide. The release of galactose and glucose monomers by hydrolysis of the β-glycosidic bond allows

Then, the GLUT2 transporter carries these monosaccharides into the blood [2, 3]. The β-glycosidic bond in lactose molecules is hydrolyzed in the small intestine by a β-galactosidase. There are three types of these enzymes in human tissue: (a) the β-galactosidase in the lysosomes, (b) the β-galactosidase in the cytosol of cells, and (c) the β-galactosidase in the small intestine. It is worth to mention that the human intestinal β-galactosidase has similarities to intestinal lactases reported in rabbit (83%) and rat (77%). However, this enzyme has no sequence homology with the other two types of β-galactosidases in human tissue, the β-galactosidase in bacteria,

The intestinal β-galactosidase (like other carbohydrate-hydrolyzing enzymes) is situated close to the brush border on the upper surface of enterocytes on the microvilli. Hundreds of tiny finger-like structures (villi) protrude from the small

or different kinds of β-galactosidases found in eukaryotic cells [2, 4].

dependent transporter SGLT1.
