I.	Milk Lipids

	1.	Triglycerides

		A.	Also Triacylglycerides
		B.	98% of total milk fat by weight

	2.	Other Milk Lipids

		A.	Diacyglycerides, Monoacylglycerides, Phospholipids, 
			Cholesterol, Glycolipids, Free Fatty Acids

II.	Sources of Fatty Acids

	1.	Triglycerides are synthesized in Mammary Gland
		a.	Fatty acids used from two sources
			i.	Breakdown of Blood Lipids
			ii.	De novo Synthesis in Mammary gland

	2.	Blood Lipids
		
		A.	40-60% of fatty acids
			a.	Primary source of Long Fatty Acids
				i.	>C16
		B.	Primarily Derived from VLDL
			a.	Synthesized in intestine or liver
			b.	90-95% Lipid in core
				i.	55-60% TGs 
			d.	5-10% Protein on outer surface
		C.	Chylomicrons also a source
			a.	Contain ingested fatty acids from intestine
		D.	Enter Mammary Cell via Lipoprotein Lipase
			a.	Hormone-sensitive
				i.	Prolactin in Non-Ruminants
				ii.	PL and ST in Ruminants
				iii.	Glucagon, ACTH, etc.
			b.	HDL and LDL "remnants" may enter 
				through specific receptors
				i.	Endocytosis
			c.	Source of fatty acids and glycerol

	3.	De Novo Fatty Acid Synthesis

		A.	Synthesis of Short and Medium Chain Fatty Acids
			a. 	Short=C4 to C8
			b.	Medium=C10 to C14
		B.	Occurs in the Cytoplasm 
			of the Mammary Epithelial Cell
		C.	Carbon Sources
			a.	Acetate
				i.	Most important
			b.	Beta-hydroxybutyrate (BHBA)
			c.	Absorbed from blood
		D.	NADPH needed
			a.	Reducing Equivalent
		E.	Two Key Enzymes
			a.	Acetyl-CoA Carboxylase
				i.	Rate Limiting Enzyme for FA synthesis
			b.	Fatty Acid Synthetase
				i.	Large Complex of Enzymes
				ii.	Responsible for Chain Elongation
		
III.	Fatty Acid Synthesis

	1.	Steps
		
		A.	Activation
			a.	Acetyl-CoA carboxylation
			b.	Converts Acetyl CoA to Malonyl CoA
		B.	Elongation
			a.	Malonyl-CoA pathway
				i.	Catalyzed by FA Synthetase
				ii.	6 Enzymes
			b.	Substeps
				i.	Condensation
				ii.	Reduction
				iii.	Dehydration
				iv.	Reduction
		C.	Repeat the Cycle

	2.	Malonyl-CoA Pathway

		A.	Reaction for Palmitate
			a.	Acetyl-CoA + 7 Malonyl CoA + 14 NADPH
						YIELDS
				Palmitate + 7 CO2 + 14 NADP + 8 CoA
		B.	Catalyzed by Fatty Acid Synthetase

	3.	Beta-Hydroxybutyrate (BHBA)

		A.	Enters Cycle as a Primer only
			a.	Contributes up to 50% of first 4 carbons
		B.	Not used for FA synthesis at later cycles

	4.	Summary
		
		A.	Occurs in Cytoplasm
		B.	Intermediates are linked to ACP
		C.	Enzymes of FA synthesis are linked in a complex
		D.	Elongation occurs by 2 Carbons/cycle
			a.	Source is always acetyl CoA
		E.	Reducing Agent is NADPH
		F.	Elongation stops at C16
		
	5.	Ruminant vs. Nonruminant Fatty Acid Synthesis

		A.	Origin of Acetyl CoA and NADPH different
		B.	Ruminant
			a.	Acetyl CoA
				i.	Acetate
				ii.	BHBA
			b.	NADPH
				i.	Pentose Shunt
				ii.	Isocitrate Dehydrogenase
		C.	Nonruminants
			a.	Acetyl CoA
				i.	Glucose
			b.	NADPH
				i.	Pentose Shunt
				ii.	Malate-NADP dehydrogenase
				iii.	Citrate Lyase	

IV.	Lipid Droplet Formation and Secretion

	1.	Formation
		
		A.	Synthesis of triglycerides occurs 
			on outer surface of SER
			a.	Esterification of fatty acids 
				to glycerol backbone
				i.	Acyl transferase
				ii.	Usually  alpha-glycerol phosphate
					(glycerol-3-phosphate)
					--From Triose Phosphate of Glycolysis
				iii.	Also can come from Lipase products
		B.	TGs coalesce to form micro lipid droplets
		C.	Bleb off of SER surface into cytoplasm

	2.	Secretion

		A.	Micro lipid droplets fuse to form large droplets 
			(Cytoplasmic lipid droplets)
			a.	Surrounded by nonbilayer coat
				i.	Protein
				ii.	Gangiosides (Polar lipid)
				iii.	Needed for fusion of lipid droplets
					--Calcium also involved
		B.	Move toward apical membrane
		C.	Droplet forces way through apical membrane
		D.	Pinches off containing portion of plasma membrane
		E.	Milk fat globule enters alveolar lumen
			a.	Membrane bound
			b.	0.5 to 15 mm in diameter
			c.	Proteins are trapped within globule
				--Important for whipping properties of cream