MILK COMPOSITION:LIPIDS OR FATS

Leo TimmsIowa State University

Why is fat important? Concentrated source of energy

2.25x as much E as glucose Provide insulation

Maintain body temperatureProtect vital organs

Membrane synthesis Vitamin transport

MANY MORE!!!

MILK COMPOSITION

LIPIDS OR FATS• Mostly triglycerides

• most variable component!!!- across and within species!

• <1% - >50% fat: species differences• variability in fatty acid profiles (handout)• Fatty acid content is labile: diet changes

• major TASTE component in milk

LIPIDS OR FATS . Wt % of lipids .Lipid Cow Human RatTriglyceride 97-98 98.2 87.5

Diglyceride .25-.48 .7 2.9Monoglyceride .02-.04 T 0.4Free fatty acids .1-.4 .4 3.1Phospholipids .6 – 1 .25 0.7Cholesterol .2 - .4 .25 1.6

Triglyceride

Glycerol + 3 Fatty Acids Triglyceride

Connected by an ester bond

Fatty acids can be the same or mixed

FATTY ACIDS• Long chain: > C14 (14 carbons)

- diet - adipose• Medium chain: C10-C14 (10-14 carbons)

• Short chain: < C8 (< 8 carbons) * made in the mammary gland

- glucose (NR) - acetate/butyrate (rum)

• Saturated: no double bonds• Unsaturated: double bonds

LIPIDS OR FATS

LIPIDS OR FATSWhere do milkfat precursors come from?

1. Blood lipids: long chains• diet and adipose tissue

• lipoproteins (liver)• chylomicrons (gut)

• free fatty acids / ketones2. Glucose: glycerol

fatty acids (non ruminants)3. Acetate and butyrate (ruminants)

LIPIDS OR FATS

FATTY ACIDS• Long chain: > C14 (14 carbons)

- diet - adipose• Medium chain: C10-C14 (10-14 carbons)

• Short chain: < C8 (< 8 carbons) * made in the mammary gland

- glucose (NR) - acetate/butyrate (rum)

• Saturated: no double bonds• Unsaturated: double bonds

FATTY ACIDS• What do the fatty acids of triglycerides look like that get to the mammary gland?

• long chain fatty acids (diet / adipose)• diet = mammary gland (non-ruminants)• diet is different than mammary gland in ruminants

• Polyunsaturated fatty acids from feeds are hydrogenated (saturated) in rumen

• Rumen by pass fats = mammary gland!

LIPIDS OR FATS

GETTING FATS INTO THE GLAND

• 35-50% of triglycerides (chylomicrons/ lipoproteins taken up by mammary gland)

• Triglycerides must be broken down to get into mammary gland.

• Lipoprotein lipase: capillary walls• 17 -45% of milk fat from acetate• 8 - 22% of milk fat from butyrate• Ruminants: 50% diet / 50% de novo syn.• Acetate/ butyrate / ketones in milk (diag.)

Lipoprotein lipase (LPL)• Anchored to capillary walls• Hydrolyses triglycerides

– 1-3 times• All cells have equal access to free FA’s

– Therefore must circulate in blood as triglycerides

• Tissues with higher LPL activity can uptake more fat from blood

Free fatty acids

MILK FATTY ACIDS - COW

% from de novo % from VLDLFatty acid synthesis fatty acids C4 - C10 100 0 C12 80 - 90 10 - 20 C14 30 - 40 60 - 70 C16 20 - 30 70 - 80 C18 0 100

DE NOVO FATTY ACID SYNTHESIS IN THE MAMMARY GLAND

• Occurs in the cytoplasm!• Need a few things:

* short carbon chains ( acetyl - CoA)- acetate and butyrate ( ruminants)- glucose (acetate) in non ruminants* reducing equivalents ( NADPH2)

* proper enzymes: Acetyl CoA carboxylase / fatty acid synthetase

DE NOVO FATTY ACID SYNTHESIS IN THE MAMMARY GLAND Acetyl CoA (2C) + HCO3 + ATP =Malonyl CoA (3C) + ADP + P + H+

(acetyl CoA carboxylase - rate limiting)

Acetyl ACP (2 C) + Malonyl ACP (3C)next 4 steps - fatty acid synthetase **Large single cpmplex of enzymes

DE NOVO FATTY ACID SYNTHESIS IN THE MAMMARY GLAND

Acetyl ACP (2 C) + Malonyl ACP (3C) condensation ACP + CO

reduction NADPH - NADP dehydration H2O reduction NADPH - NADP 4 carbon chain (Butyrate) • Add 2 carbons at 1 time: C2 + C3 = C4 + CO2• Lots of energy: 1 ATP and 2 NADPH / 2 carbons• fatty acid chain attached to acyl carrier protein

MILK FATTY ACIDSC4:0 Butyric acid C16:0 Palmitic acidC6:0 Hexanoic C18:0 StearicC8:0 Caprylic C18:1 OleicC10:0 Caproic C18:2 Lineoleic (ess.!*)C12:0 Auric C18:3 Linolenic (ess.)C14:0 Myristic

• Most species: lots of palmitate / oleic• More short chains in ruminants• Conjugated linoleic acid (CLA)• Other species diff: goats C8 + C10

MILK TRIGLYCERIDE SYNTHESIS

• Occurs at cytoplasmic surface of smooth endoplasmic reticulum

• Glycerol + fatty acids (esterified)• Esterases (enzymes)• Fatty acid specificity to C site• Fatty acids coalesce to form droplets• Grow and move to apical membrane• Secreted with membrane (protein coating)

MILK COMPOSITIONApical membrane

Basementmembrane