BIOCHEMISTRY AND HUMAN NUTRITION

Module 4. Human nutrition

Lesson 31
ELEMENTARY KNOWLEDGE OF MILK SYNTHESIS IN MAMMARY GLAND

31.1 Introduction

• Precursors of milk come from the bloodstream. It is estimated that the production of 1 liter of milk requires 500 liter of blood moving through the mammary gland to provide the milk precursors.
• Some materials in the milk come unchanged from blood. These include minerals, some hormones and some proteins (e.g. immunoglobulins).
• Only precursors of milk protein and carbohydrates are present in blood. The primary substrates extracted from blood by the lactating mammary gland include glucose, amino acids, fatty acids, ß-hydroxybutyrate, and salt.

31.2 Milk Fat

• Cow's milk contains 3.5 to 5% fat.
• About 97 to 98% of the fat is triglycerides (also known as triacylglycerols or triacylglycerides) and phospholipids constitute about 1% (Table. 31.1). Palmitic (C16:0) and oleic (C18:1) acid are the main fatty acids in milk fat.
• Milk fat contains low levels of short chain fatty acid (C12 and less).

Table 31.1 Milk lipid composition of dairy cows

31.2.1 Biosynthesis of milk lipids (Triglycerides)

• Sources of Milk Fatty Acids

The fatty acids used to synthesize milk fat (triglycerides) come from two sources:

• Blood lipids: Derived from digestion and absorption of dietary fat and from mobilization of fatty acids from adipose tissue. Most of the fatty acids derived from blood plasma are of dietary origin (> 80%).
• De novo synthesis within the mammary epithelial cells (synthesis of new molecules of fatty acids from precursors absorbed from the blood) Acetate and ß-hydroxy-butyrate are the major carbon sources of fatty acid biosynthesis in the mammary gland. Almost all C4 to C14 fatty acids (short and medium-chain fatty acids) are synthesized de novo. Short chain fatty acids of various lengths are synthesized by the step-wise addition of acetate to ß-hydroxy-butyrate.

• Milk fat triglycerides are synthesized in the cytoplasm surface of the smooth endoplasmic reticulum of mammary epithelial cells. Milk lipids (triglycerides) are synthesized from fatty acids and glycerol through the α-glycerol phosphate pathway (Figure 31.1). Acetyl CoA carboxylase is the key milk biosynthesis enzyme and its activity increases considerably during lactogenesis (copious milk secretion).
• Two acyl CoA molecules react with α-glycerol-3-phosphate to form phosphatidic acid, which upon removal of the phosphate, leaves a 1,2 diacylglycerol. An additional long chain acyl CoA adds the final fatty acid, with the formation of triacylglycerol and CoA.

Fig. 31.1 Biosynthesis of milk tryglycerides

31.3 Milk Proteins

• The nitrogen content of milk is distributed among three major groups:

• Caseins (76% of total milk nitrogen)
• Whey protein (18% of total milk nitrogen)
• Non-protein nitrogen (6% of total milk nitrogen)

• True proteins (i.e. excluding NPN) are classified into three fractions:

• Caseins present in micelles
• Whey proteins present in solution
• Fat globule membrane proteins on the surface of fat globules.

• Milk proteins contain more amino acids than any other natural food.

These include:

• Casein proteins; α-, β-, and k-casein
• Lactoglobulin; b-lactoglobulin (~50% of whey proteins)
• Lactalbumin. a-lactalbumin (~25% of whey protein)

• A second group blood proteins (e.g. immunoglobulins) and some proteins synthesized in the plasma cell adjacent to the secretory epithelium, enter the mammary gland and appear in the milk unchanged.

31.3.1 Biosynthesis of milk protein

• Milk proteins are synthesized from amino acids present in the mammary secretory cell. The biosynthesis takes place in the ribosome, which is attached to the rough endoplasmic reticulum. Steps of biosynthesis are similar to those of any other protein:

1. Transcription: A strand of messenger RNA (mRNA) is formed from DNA. It carries the code of a specific protein. The mRNA is located in the ribosome, which is attached to the rough endoplasmic reticulum.
2. Activation: Amino acids in the cytoplasm are activated by reaction with ATP and attachment to transfer RNA (tRNA). The tRNAs are specific for each amino acid.
3. Translation: Takes place in the ribosomes. The mRNA contains codes for amino acids. Each code consists of three nucleotides and is known as a codon. Located in the tRNA a trinucleotide anticodon that recognizes it. As each codon in the mRNA comes in position, the appropriate amino acid-tRNA complex moves in the amino acid joined the previous one in the chain.

31.3.2 Biosynthesis of milk carbohydrates (Lactose)

• Lactose is the most constant constituent in bovine milk (about 4.5%). The main function of lactose is to maintain the osmolality of milk during the formation and secretion process.
• Glucose is essential for lactose synthesis and cannot be replaced by any other sugar. About 45-60% of blood glucose in ruminants is synthesized in the liver from propionate by a process known as gluconeogenesis. Blood glucose levels in ruminants are about half those found in non-ruminants.

31.3.3 Lactose biosynthesis

• The site of lactose synthesis is the membranes of the Golgi apparatus. Glucose is the only precursor and two molecules of glucose are required for each molecule of lactose. One molecule of glucose is converted to galactose. The enzyme catalyzing this conversion appears just before parturition and its activity increase dramatically at the onset of milk synthesis in lactation (Lactogenesis).
• The condensation of glucose and galactose involves the enzyme lactose synthase. The enzyme composed of two proteins (galactocyl trnasferase and -lactalbumin) that must be together for lactose biosynthesis to take place. Therefore, the rate of lactose biosynthesis is greatly influenced by the availability of α-lactalbumin from the rough endoplasmic reticulum. (Fig. 31.2)
• Lactose is a nonpermeable disaccharide, which cannot diffuse out the Golgi membrane or out of the secretory vesicles' membrane. This is important for milk synthesis because it is the synthesis of the nondiffusible lactose, which results in water being drawn into the Golgi. Water is osmotically drawn into the Golgi to try to dilute the lactose.

Fig. 31.2 Lactose synthesis

31.4 Secretion of Milk Constituents

• The individual component of milk are kept separate inside the secretory cell and therefore, milk is not formed until the individual components reached the lumen where they are mixed together.
• Milk protein synthesized in the rough-endoplasmic reticulum where it is incorporated into the Golgi vesicles (vacuoles). Other non-fat components including lactose and salts are also incorporated into the Golgi vesicles. The secretory vesicles separate from the Golgi apparatus and move towards the apical region of the cell where the membrane surrounding each vesicle fuses with the plasma membrane and the content is discharged into the lumen.
• Milk fat or lipids take a separate secretion pathway than that taken by non-fat milk components (i.e. protein and lactose). Lipid molecules increase in size as they move from the endoplasmic reticulum towards the apical membrane where they push through and break away as globules engulfed in an envelope made of apical plasma membrane. The apical membrane is composed of lipids, which come from the walls of the secretory vesicles carrying the non-fat components of milk to the apical membrane. The milk fat globule is membrane-surrounded and has a number of membrane associated proteins, these proteins and others trapped during the process of cream separation are important for the whipping properties of cream.