Effects of exchanging dietary stearic and palmitic acids on plasma lipids and in vivo lipoprotein me

1
(No Transcript)
2
Vitamin A

• Vitamin A (or preformed vitamin A) is retinol
  (alcohol) or retinal (aldehyde). Retinoic acid
  derived from retinal
• Provitamin A is b-carotene
• Carotenoids occur widely in plants but only
  10 have viatmin A activity (a-carotene,
  b-carotene, g-carotene)
• b-carotene has highest vitamin A activity
• Free retinol not found in foods occurs as
  retinyl ester (retinol fatty acid). Common one
  is retinyl palmitate

3
Digestion, absorption and transport 1

• Carotenes RE complexed with protein in foods
• So ? proteases in stomach and small intestine ?
  removes proetin. Esterases remove fatty acids
• Resulting carotenoids retinols ? micelles ?
  diffuse into microvilli ? into intestinal cells
• 70-90 of dietary retinol absorbed. Carotenoid
  absorption inversely related to intake (for b-C
  20-50)
• In intestinal cell, enzymatic conversion of b-C ?
  retinal
• ? retinol. Some retinol can also form retinoic
  acid by oxidation
• Ultimately, carotenes and RE ? CM ? circulation

4
Absorption, Transport and Storage-2

• CM transported as RE, retinol and carotenoids to
  peripheral tissues (spleen, adipose, muscle,
  lungs etc) and liver
• From liver carotenoids can form retinol, stored
  or be released as VLDL (and so to rest of body).
  Excess carotenoids not stored in liver dumped in
  body lipids
• RE reaching liver hydrolysed. Resulting retinol
  binds a cellular retinol-binding protein (CRBP)
  which directs its ultimate metabolism
• Retinol stored in liver, typically stored after
  re-esterification. Resulting RE stored in
  specialized liver cells

5
Absorption, Transport and Storage-3

• Use of stored RE. Hydrolysis produces free
  retinol which combined with retinol binding
  protein (RBP) to form holo-RBP
• In plasma holo-RBP also combines with
  transthyretin (TTR) - used to be called
  prealbumin and thyroxine so you have a
  retinol-RBP-TTR complex
• Complex circulates in plasma delivering retinol
  to target tissues whereby retinol is cleaved and
  enters cell or combines with surface receptor
• RBP left over cannot bind with TTR and is
  catabolized by the kidney

6
Vitamin A functions

• Visual cycle
• Cellular differentiation
• Growth
• Cell surface role glycoproteins
• Other functions

7
Visual cycle

• Retinol enters retina as the RBP-TTRv complex )
  can be stored here as RE)
• Re hydrolyzed releasing retinol
• Retinol oxidized to trans retinal which
  isomerizes to the cis retinal
• 11 cis retinal binds to lysine residues of opsin
  protein giving rhodopsin (latter in disks)
• Quantum of light causes rhodopsin to bleach
  series of breakdown reactions resulting in
  trans retinal signal to brain (action
  potential) involving a neurotransmitter (cGMP or
  calcium)
• Trans retinal reconverts to cis retinal and
  rebinds rhodopsin

8
(No Transcript)
9
(No Transcript)