Vitamin A and carotenoids Flashcards by Cindy Davis

What are the forms of preformed vitamin A (retinoids)

retinol (alcohol)

oxidized to

retinal (aldehyde)

oxidized to

retinoic acid (carboxylic acid)

retinyl esters (retinol attached to fatty acid, most often palmitate)

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What is the basic structure of retinoids

a beta-ionone ring
a polyunsaturated side chain (made of isoprenoid units)
a polar functional group

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What is the role of retinyl esters

the body stores vitamin A in this form

2. it is the form of preformed vitamin A most commonly found in food.

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What is the role of 11-cis-retinal?

It is the form required to form rhodopsin in rod cells for night vision

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What forms of vitamin A are utilized in gene expression?

all-trans-retinoic acid

9-cis-retinoic acid

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What synthetic analogues of vitamin A are available?

all trans retinyl palmitate

all trans retinyl acetate

(these can also be natural forms)

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Name the carotenoids with physiological significance in the body?

Beta-carotene
Alpha-carotene
beta-cryptoxanthin
lycopene
lutein
canthaxanthin
zeaxanthin

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Which carotenoids can be converted to vitamin A?

Beta-caortene
alpha-carotene
beta-cryptoxanthin

These carotenoids have provitamin A activity. Beta-carotene has the greatest provitamin A activity of these.

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Which carotenoids have physiological significance but can not be converted to vitamin A?

lycopene
lutein
zeaxanthin
canthaxanthin

These carotenoids do not have provitamin A activity.

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How many total carotenoids are there?

How many have vitamin A activity?

Over 600 carotenoids have been identified.

Only about 10% exhibit vitamin A activity.

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What measures are used for vitamin A and what are the conversion rates?

Retinol Activity Units (RAE)
1 RAE = 1 mcg retinol = 3.33 IU

International Units (IU)
1 IU = 0.3 mcg retinol = 0.3 mcg RAE

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What are the RAE equivalencies for retinol, beta-carotene, and alpha-carotene?

1 RAE = 1 mcg retinol
1 RAE = 12 mcg beta-carotene
1 RAE = 24 mcg alpha-carotene or beta-cryptoxanthin

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What are the IU equivalencies for retinol, beta-carotene, and alpha-carotene?

1 IU = 0.3 mcg retinol
1 IU = 3.6 mcg beta-carotene
1 IU = 7.2 mcg other provitamin A carotenoids

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What is the RDA for vitamin A for

Adult men
Adult women
Pregnant women
Lactating women

Adult men 900 RAE (3,000 IU)
Adult women 700 RAE (2,330 IU)

Pregnant women 770 RAE (2,570 IU)
Lactating women 1,300 RAE (4,330 IU)

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What is the UL for vitamin A?

3,000 RAE preformed vitamin A (10,000 IU retinol)

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What foods are good sources of preformed vitamin A?

Beef liver
Chicken liver
Cod liver oil

Milk, whole
Butter
Eggs

What are good food sources of carotenoids?

orange, yellow, and red fruits and vegetables

carrots
squash
papaya
watermelon
tomatoes

green vegetables also contain some carotenoids

Describe the process for digestion of vitamin A.

retinyl esters are freed from attached protein by pepsin in stomach and proteolytic enzymes in the SI
Separate fatty acids from retinyl esters. Bile acids in duodenum emulsify fats so hydrolase and lipase enzymes can act on them.
Micelles form including retinol and carotenoids and are absorbed by passive diffusion across the brush border in the duodenum and jejunum and into the enterocytes.

What is the absorption rate for vitamin A?

70% to 90% of vitamin A is absorbed as long as meal contains some fat (>=10g)

What is the absorption rate of beta-carotene and other carotenoids?

Beta-carotene 20% to 50% (per Gropper text) (14% per Krause’s)

Other carotenoids - less than beta-carotene

in uncooked vegetables the amount is <5%

What inhibits the absorption of vitamin A and carotenoids?

Vitamin A -
1. low dietary fat (<5%)

Carotenoids -

fiber, especially pectin
excess intake of vitamin E

How is retinol metabolized in the enterocyte?

CRBPII attaches to retinol
LRAT esterifies a fatty acid creating CRBPII-retinyl palmitate
Export of CRBPII retinyl palmitate in chylomicrons into lymph system then into blood stream

How is beta-carotene metabolized in the enterocyte?

Beta-carotene can be converted to retinal in the enterocyte

15,15’ mono-oxygenase converts one moledule of beta-carotene into two molecules of retinal
the retinal can be converted to retinoic acid or
the retinal can be converted to CRBPII-retinol

(about 15% of beta-carotene can leave cell intact)

What metabolic products of retinol can be produced in the enterocyte?

retinoid beta-glucuronide (excreted in the bile)
retinoyl beta-glucuronide (can enter circulation through the portal vein)
9-cis retinoic acid (for gene expression)

11-cis retinal (required for vision)

What are the 4 paths that retinol can take in the hepatic parenchymal cells?

1. Storage: CRBP-retinol for storage in hepatic stellate cells 2. Transport to other tissues: Attached to RBP to be released in the blood stream as holo-RBP. In the blood stream transthyretin and T4 are attached forming the trimolecular complex. 3. Excretion in bile: conjugated to glucuronic acid forming retinoid beta-glucuronide for excretion in bile. 4. Conversion to retinoic acid: which can be converted to - CRABP-retinoic acid (functions in nucleus in gene expression) - retinoyl beta-glucuronide - 4-oxoretinoic acid (involved in synthesis of gap junctions)

What are the 3 paths that carotenoids can take in hepatic parenchymal cells?

1. cleavage into retinal 2. incorporation into VLDL or other lipoproteins for delivery to tissues 3. storage in the liver

Where is vitamin A stored in the body?

Adipose tissue stores 15% to 20% of the body's vitamin A Liver (stellate cells) 80% to 95% of body's retinol stored as retinyl esters

What are the main functions of vitamin A in the body?

``` Vision (11-cis retinal) Gene expression (all trans retinoic acid or 9-cis retinoic acid) Cellular differentiation (retinoic acid) Growth Reproduction and embryo morphology Bone development Immune system function ```

Describe how vitamin A is used in vision.

1. retinol (holo-RBP-TTR) is received by pigment epithelial cells in the retina 2. the retinol is converted to 11-cis retinal 3. 11-cis-retinal is transported across interphotoreceptor space by IRBP to the rod cell 4. In the rod cell a. 11-cis-retinal attaches to opsin to form rhodopsin b. when light is absorbed by rhodopsin, 11-cis-retinal is converted to all trans retinal (bleaching) c. all trans retinal separates from opsin d. all trans retinal is reduced to all trans retinol and exported out of rod cell, across interphotoreceptor space attached to IRBP 5. In pigment epithelium all trans retinol is converted back to 11-cis-retinal

Describe how vitamin A functions in gene expression.

1. CRABPII or other intracellular lipid binding protein carries retinoic acid into nucleus 2. Vitamin A binds to retinoic acid receptors (RAR) or retinoid X receptors (RXR) in the retinoic acid response element (RARE) in the promoter region of the gene 3. the receptor undergoes a conformational change that results in release of corepressors promoting transcription of the genes

Differentiation of what cell types is regulated by vitamin A (retinoic acid)?

keratinocytes (immature skin cells) myeloid precursor cells (blood cell precursor) cancer cells

How is bone growth affected by vitamin A excess or deficiency?

Deficiency of vitamin A results in 1. excessive bone deposition by osteoblasts 2 reduced bone resorption by osteoclasts Excess vitamin A results in 1. reduced bone mineral density and increased risk of fracture 2. stimulates bone resorption by osteoclasts 3. inhibits bone deposition by osteoblasts.

Which vitamin stimulates phagocytic activity and antibody production? D A E K