lecture 6

• milk biosynthesis- nutrients are obtained by the circulation (capillaries) within the mammary gland
• veins carry waste and carbon dioxide to the heart
• arteries carry gas, oxygen, and nutrients away from the heart and to the mammary gland
• polarization describes the idea that the nucleus should be closer to the basolateral membrane vs the apical membrane
• the apical membrane contains more fat globules secreted as lipid droplets
• milk is described as being nature's most perfect food
• main components of milk include water, protein, fat, carbohydrates, vitamins, minerals, and cells
• water is milk is secreted through apical transport
• protein in milk is secreted mostly through exocytosis
• fat in milk is secreted through lipid secretion via the apocrine
• carbs in milk are secreted through exocytosis
• vitamins in milk are either fat and/or water soluble
• immunoglobulins are secreted via transcytosis
• the milk space allows for 4 types of secretion: exocytosis, lipid secretion, apical transport, and transcytosis
• secretory epithelial cells synthesize and secrete carbs, proteins, and lipids
• lactose is the main carbohydrate in milk and is the major determinant of milk volume
• lactose is a disaccharide (2 sugars) formed by 1 molecule of galactose and 1 molecule of glucose bonded by a beta 1,4 glycosidic bond
• glucose can come from metabolic pathways or the diet
• the two metabolic pathways that synthesize glucose include gluconeogenesis and glycogenolysis
• gluconeogenesis is the synthesis of glucose from non-carbohydrate precursors (lactate, amino acids, glycerol, propionate), specifically in the liver
• glycogenolysis is the breakdown of glycogen to glucose that can then be used as an energy source; in liver, happens when fasting
• in non-ruminants, dietary carbohydrates include starches and sugars that are digested to glucose, absorbed in the small intestine, and then delivered to the mammary gland via circulation
• in ruminants, dietary carbs include forage and grains (cellulose, hemicellulose, and starch) which is broken down via ruminal fermentation with volatile fatty acids (like acetate, propionate, and butyrate)
• propionate is the most important volatile fatty acid because it contains 3 carbons, and glucose has 6 carbons, so it only takes 2 propionate to make one glucose (most efficient)
• lactose synthesis can be broken into 3 steps: galactose and glucose arrive at the site of reaction (golgi), synthesis of lactose with lactose synthase enzymes, and removal of byproducts back to cytoplasm
• galactose and glucose to site of reaction (golgi): 2 glucose molecules used per 1 lactose molecule; glucose #1 is transported to golgi via GLUT-1 transporter (passive transport, facilitated diffusion, no energy required)
• galactose and glucose to site of reaction (golgi): 2 glucose molecules used per 1 lactose molecule; glucose #2 is converted to UDP-galactose and is then actively transported to the golgi (requires energy)
• in step 2 (lactose synthesis): lactose is synthesized in the goli apparatus by the enzyme lactose synthase complex
• the lactose synthase complex is an enzyme formed by 2 proteins: galactosyltransferase (not unique to mammary gland, glycoprotein synthesis) and alpha-lactalbumin (co-factor, unique to mammary gland)
• step 3, removal of byproducts: UDP cleaved to UMP + Pi by NDPase enzyme, UMP actively transported from golgi to cytosol, UMP may be converted back to UTP at the expense of 2 ATPs
• UTP + glucose-1-P ---> UDP-glucose + P
• UDP-glucose ---> UDP-galactose
• glucose + UDP-galactose ---> lactose + UDP
• lactose is the main osmoregulator of milk volume: 1. increased concentration of galactose in golgi, 2. increased osmolality within the golgi and secretory vesicles, 3. creates a gradient that promotes diffusion of water into these structures since lactose cannot simply leave the golgi
• the semi-permeable membrane allows for some transfer of water, but not solutes; maintains a gradient
• milk protein is composed of 80% casein and 20% whey
• renin separates casein and whey
• the pH of milk proteins is ~ 4.6
• casein includes alpha, beta, kappa, and gamma
• whey includes alpha-lactalbumin (4%), beta-lactoglobulin (10%), and serum albumin immunoglobulins (5%)