Milk Synthesis

Suporn Katawatin

Mammary Gland: the special gland

Offer no advantage to dam

Huge physiological demand from dam

Get high priority on nutrients, on expense of

dam health

Demand metabolic change

Increase metabolic rate

Increase blood flow

Increase nutrient supply

Inability to meet demand result in metabolic disorder: milk fever (hypocalcemia), ketosis (hypoglycemia)

Three major functions in milk synthesis

First, breaks down substrates to provide energy: mitochondria

Second, synthesize components of milk: lipids, casein, lactose

requires substrates, enzymes, and environment

Third, regulate non-mammary synthesized constituents: water, vitamins, and minerals

blood precursors for milk

Glucose

Acetate

β-hydroxybutyrate

triglyceride fatty acids

Amino acids

Two major sources of energy are glucose and

acetate

Energy: one of three pathways

Embden-Meyerhoff glycolytic pathway

Citric acid cycle: final common pathway of

metabolism

Pentose phosphate pathway, or shunt

Energy Production in Mammary Gland

Glucose is utilized by mammary cellsGlycolysis (Embden-Meyerhof glycolytic pathway) allows cytoplasmic anaerobic conversion of glucose to pyruvic acid and ATP, then coupled to citric acid cycle, and yield 38 ATP Utilization of pentose phosphate pathway or shunt to yield NADPH+ H+ to yield 35 ATP

In cow: of blood glucose taken up by mammary cellSixty to 70% utilized for lactose synthesis in alveolar cell Twenty to 30% oxidized via the pentose phosphate shunt for fatty acid synthesis Less than 10% utilized via glycolysis and Krebs Cycle

Milk Synthesis Process

Milk synthesis processes

Precursors leave blood and enter extracellular fluid,

through basolateral membrane and to appropriate

synthetic pathway

Immunoglobulins transported intact through cell

Milk synthesis processes

5 routes to enter alveolar cells :

For uptake amino acids

For uptake sugars and salts

For uptake milk fat precursors

For uptake preformed proteins (immunoglobulins)

paracellular pathway

Alveolar lumen

pathway

Precursors from blood

Products form in lumen

Salts

leukocyte

Salts

leukocytes

aa

aa

glucose

Lactose glycerol

FA

FA, glycerol acetate OH-

butyrate

lipids

immunoglobulin

immunoglobulin

Tight junction

caseinFat droplet

Tight junction

Fat

Protein

Fat droplet

Basal membrane

Lumen

Amino acids to proteins

Aa absorbed via basal membrane by specific aa

transport systems

Inside cell, aa covalently bound together to form

proteins at polysomes on RER

transferred from RER to Golgi apparatus to post-

translational processing and packaging as secretory

vesicle

Proteins synthesized include casein, β-lactoglobulin,

α-lactalbumin and membrane bound proteins or

enzymes

Amino acids to proteins

Secretory vesicles of milk proteins (also

lactose) move to apical membrane

secretory vesicle fuses with inner

surface of apical membrane, opening

and vesicle contents discharged into

alveolar lumen

Glucose to lactose

Glucose enters cell via basolateral membrane by specific transport mechanismSome converted to galactose, both glucose and galactose enter Golgi and forming lactoselactose drawing water into cell, Golgi, and becoming part of milkThus, Golgi apparatus involved in processing of milk proteins, synthesis of lactose, and osmotic draw for waterLactose secreted via secretory vesicles with milk proteins

Milk fat precursors to milk fat

Precursors taken up via basolateral membrane

Acetate and β-hydroxybutyrate, precursors of FA

synthesis in ruminants mammary cells

Preformed fatty acids, glycerol, and

monoacylglycerides absorbed into synthesis of

triglycerides of milk

Milk fat precursors to milk fat

Milk fat triglycerides synthesized on SER and form

small droplets

Small droplets fuse and moves toward apical

membrane and pinches off to alveolar lumen

Thus, inside cell nonmembrane-bound lipid

droplet but in alveolus lumen, milk fat globule

surrounded by membrane

Transport of Milk Components Not Synthesized in the Epithelial Cells

Immunoglobulins

Ig bind to specific receptors on basolateral

"into" cell in endocytic vesicles

transported to fuses with inner apical membrane

and releases into alveolus lumen

no serum albumin receptor, however, serum albumin may internalized along with Ig

Paracellular Pathway

way through Tight junctions between

epithelial cells

little or no "flow" of anything via Tight

junctions let except water and some ions

Tight junctions become ‘leaky’ during

mastitis or involution, or when oxytocin

causing milk ejection

Paracellular Pathway: leukocytes

leukocytes pass between epithelial cells and "break open" tight junctions

allows other extracellular components like salts to diffuse into the lumen and milk components to diffuse out of the lumen into the extracellular fluid

More in Mastitis Module

Paracellular Pathway

allow lactose and K+ move from lumen into

extracellular space, and Na+ and Cl- move opposite

Results in changing electrical conductivity of milk

(as used in detecting mastitis), increase lactose

and other milk-specific components in blood

Lactose found in urine of a cow during peripartum

period

Milk proteins found in cow's blood during

lactation and early involution

Synthetic Activity in Mammary Cells

Cells within an alveolus synchronized in

synthetic activity

some alveoli cells full of lipid droplets

and secretory vesicles, while other

none of those structures

Synthetic Activity in Mammary Cells

Secretory activity occur in two phases:

Formation of intracellular secretory structures like lipid

droplets and secretory vesicles

tall and columnar cells

Release products into lumen

Cuboidal cells , lumen fills with milk

Intracellular synthesis may decrease during this time

Synthetic activity in mammary cell

incubate mammary tissue with labeled aa,

Tracer moves through cell as a pulse

Stays in cytoplasm ~3-15 min. [newly synthesized proteins]

Appears in Golgi within 15-30 min. [newly synthesized proteins being processed]

increased in lumen 30-60 min. later [secreted proteins]

Synthetic activity in mammary cell

inject radiolabeled precursors of milk components into

animal

found in milk at varying times

Those enter by equilibrating across the apical or Golgi

membranes (Na+, K+, Cl-) take ~1 hr to reach max.

specific activity in the milk

Those enter by synthesis in the Golgi (lactose, casein,

Ca, citrate, phosphate) take 2-3 hr.

Those enter as part of milk fat synthesis take 5-7 hr.

Further readings

DASC Ch.VII:

Milk Biosynthesis: MilkBiosynthesis.doc

Biochemistry of the Mammary Gland:

MamBiochem.doc