Download Handout 10 - Fat soluble vitamins

Handout 10
Fat Soluble Vitamins
ANSC/NUTR 618
Lipids & Lipid Metabolism
Fat Soluble Vitamins
I. Biochemistry of fat-soluble vitamins
A. Vitamin A and carotenoids
1. Structure
a. Dietary form – retinol
b. Can be oxidized to retinal and
retinoic acid (active form which also is
the form that is excreted).
c. Stored and transported in
chylomicrons as retinyl-palmitate.
2. Absorption and storage
a. Retinyl esters are hydrolyzed pancreatic retinyl ester hydrolase to retinol,
carotenoids, and free fatty acids.
b. Retinol and carotenoids are incorporated into mixed micelles and absorbed by
diffusion into the enterocytes (80-90% efficient or retinol, 50% efficient for
carotenoids).
c. Retinol is esterified with palmitate (to make retinyl esters) fin mucosal cells and
incorporated into chylomicrons.
1) The chylomicron remnants are taken up by the liver and retinyl esters and some
carotenoids are stored in the lipocytes of the liver.
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Handout 10
Fat Soluble Vitamins
2) Carotenoids also are taken up from chylomicrons and stored in kidney, lung,
and adipose tissue.
3) Retinol is released from the liver, binds to retinol-binding proteins (RBP), and
is transferred to target tissues.
3. Functions
a. Carotenoids serve as storage forms for retinol, which can be reversibly converted
to retinal.
b. Retinol maintains mucus cells, prevents drying of moist membranes (e.g., eye).
c. All-trans retinal is converted to 11-cis retinal, which binds with opsin to form
rhodopsin; necessary for vision.
d. Retinoic acid binds with cytosolic binding proteins, which are translocated to the
nucleus to stimulate gene transcription (e.g., retinoic acid depresses adipose tissue
differentiation).
B. Vitamin D
1. Synthesis
a. Vitamin D is
synthesized from
cholesterol.
b. One ring
structure is
opened between
the 9 and 10
carbons.
2. Absorption and storage of dietary vitamin D
a. Dietary vitamin D is incorporated into mixed micelles and absorbed by diffusion
into the enterocytes, where it is incorporated into chylomicrons.
b. Chylomicron remnants are taken up by the liver and vitamin D is stored in the
lipocytes of the liver.
c. Some portion is activated to 25-OH-D3 and released bound to vitamin D-binding
protein (DBP).
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Handout 10
Fat Soluble Vitamins
d. 25-OH-D3 is activated to 1,25-OH-D3 by the kidney in response to changing blood
Ca++.
3. Function
a. With parathyroid hormone, 1,25-OH-D3 stimulates Ca++ retention by the kidney.
b. Vitamin D3 binds to cytosolic receptor, moves into nucleus, and initiates gene
expression.
C. Vitamin E (α-tocopherol)
1. Structure (see below)
2. Absorption and storage
a. Vitamin E is incorporated into mixed micelles and absorbed by diffusion into the
enterocytes, where it is incorporated into chylomicrons.
1) The chylomicron remnants are taken up by the liver and vitamin E is stored in
the lipocytes of the liver.
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Handout 10
Fat Soluble Vitamins
2) Some vitamin E is transferred from chylomicrons to erythrocytes.
3) Vitamin E is distributed to other tissues via VLDL à LDL, with uptake of
LDL by adipose tissue and other nonhepatic tissues.
a) Turnover of vitamin E in adipose tissue and brain is slow.
b) Therefore, only is the liver is an effective storage organ for vitamin E.
3. Functions
a. Antioxidant and anti-free radical agent.
1) Prevents oxidation of unsaturated fatty acids and cholesterol in cell
membranes.
2) This prevents cell fragility.
b. Development and maintenance of nerve and muscle function.
c. Hindrance of damage by ozone, cigarette smoke.
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Handout 10
Fat Soluble Vitamins
D. Vitamin K
1. Structure
2. Absorption and storage
a. Vitamin K can be derived as 50% from plants (as phylloquinone; K1) and 50%
from bacteria (including intestinal bacteria; menaquinone; K2).
b. Vitamin K is incorporated into mixed micelles and absorbed by diffusion into the
enterocytes, where it is incorporated into chylomicrons.
1) The chylomicron remnants are taken up by the liver and vitamin K is stored in
the lipocytes of the liver (body pools are small).
2) Vitamin K is distributed to other tissues via VLDL à LDL, with uptake of
LDL by adipose tissue and other nonhepatic tissues.
3. Functions
a. Blood clotting.
b. γ-Carboxylation of glutamic acid, which increases binding of Ca++ (leads to
improved bone metabolism and connective tissue and kidney function).
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