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NHM 561 > Vitamin A > Flashcards

Vitamin A Flashcards

1
Q

Alcohol Form of vitamin A

A

Retinol

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2
Q

Aldehyde Form of vitamin A

A

Retinal

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3
Q

Storage form of vitamin A

A

Retinyl ester; has FA attached

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4
Q

Acid form of vitamin A

A

Retinoic Acid; contains carboxyl group

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5
Q

Provitamin A derivatives

A

Called provitamin A because they can be cleaved (in half), and they can be formed into retinol, include:

Beta-carotene
Alpha-carotene
Beta-cryptoxanthin

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6
Q

Carotenoids

A

Over 600 carotenoids
Included provitamin A carotenoids (beta-carotene, alpha-carotene, and beta-cryptoxanthin

Also includes: lycopene, lutein, zeaxanthin (but cannot be converted to retinol in the body)

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7
Q

RAE

A

Retinol Activity Equivalents (RAE); 1mcg RAE =

1 mcg retinol
2 mcg supplemental beta-carotene
12 mcg dietary beta-carotene
24 mcg dietary alpha-carotene or beta-cryptoxanthin

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8
Q

What form of vitamin A is found in: liver, dairy, fish (tuna)

A

Retinyl palmitate/ester

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9
Q

What form of vitamin A is found in carrots, papaya, sweet potatoes, pumpkins?

A

Provitamin A carotenoids

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10
Q

Retinoic Acid

A

the active form of vitamin A, that is responsible for controlling the expression of several genes

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11
Q

(T/F) The retinyl esters that are bound to palmitate can be hydrolyzed and converted to retinol.

A

TRUE

And from here, they can be converted to retinal. Below represents the transformation of the different forms of vitamin A in the body:

Retinyl —> Retinal <–> Retinol —> Retinoic Acid

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12
Q

Pepsin

A

Hydrolyzes protein from vitamin A in the stomach, as the first step of absorption

Remember: Typically, vitamin A will be coming in as retinyl esters so with a FA attached, and it will be bound to the animal protein, like fish.

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13
Q

What are the contents of the mixture that leave the stomach and move into the SI?

A

Large fat droplets with dietary lipids (triglycerides, cholesterol)

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14
Q

What happens in the SI?

A

The esterases and lipases are going to hydrolyze the retinyl esters into free retinol. The free retinol will get mixed up with bile, as well.

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15
Q

What is the role of bile here?

A

To emulsify the fat droplets, in the SI, making them even smaller (micelles)

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16
Q

What interfaces with the enterocytes?

A

Micelles

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17
Q

In what form does vitamin A enter the cell?

A

Retinol or carotenoids form

Enter via the micelle

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18
Q

Beta-carotene dioxygenase

A

cleaves beta-carotene into 2 RETINAL (in the enterocyte)

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19
Q

Once in the cell, what happens to the carotenoids?

A
  1. Non-provitamin A carotenoids (lycopene, etc. will go straight to a chylomicron)
  2. Provitamin A carotenoids (beta-carotene) 15% go straight to the CM; 85% will be acted on beta-carotene dioxygenase (cleaving it into 2 molecules, which will be converted to Retinal).
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20
Q

How many retinal do you end up with when beta-carotene dioxygenase cleaves beta-carotene?

A

2 retinal

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21
Q

Retinal reductase

A

Reduces retinal —> retinol (in the enterocytes)

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22
Q

What happens to retinol, in the enterocyte?

A

Remember: this can come from the retinyl ester form or carotenoid form

It will be acted on by LRAT, converting retinol —> retinyl esters

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23
Q

In what form does vitamin A leave the enterocyte, in the chylomicron?

A

Retinyl ester (when converted from retinol form)

Remember: 15% of beta-carotene will go straight to the CM, and the non-provitamin A carotenoids will go straight to CM as well.

24
Q

What is contained within the chylomicron?

A

Retinyl esters
Fat-soluble vitamins
Choleterol
Triglycerides

25
Q

LRAT

A

Lecithin retinol acyl transferase

Converts retinol —> retinyl esters in the enterocyte

26
Q

What happens once the CM leaves the enterocyte?

A

Circulates through the blood, acting as a transporter, delivering retinyl esters and carotenoids to many extrahepatic tissues (bone marrow, spleen, muscle, lungs, kidneys, adipose tissue)

27
Q

Chylomicron Remnants

A

Once the CM has dropped off most of its TGs, it becomes a remnant, containing mostly retinyl esters.

These are taken up by the liver (85-90% vitamin A)

28
Q

Where is vitamin A stored?

A

85-90% is stored in the liver

29
Q

Parenchymal Cells

A

Cells in the liver, that take up the chylomicron remnants

30
Q

Stellate Cells

A
31
Q

What 2 (binding) proteins are required for plasma retinol transport?

A

1) RBP

2) Transthyretin (TTR)

32
Q

RBP

A

Retinol Binding Protein

Primarily made in the liver, and it is a 1:1 ratio where 1 molecule of retinol combines with 1 molecule of RBP and then be released from the liver.

33
Q

Holo-RBP

A

The complex of RBP+retinol

34
Q

TTR

A

Transthyretin

In the blood, each molecule of holo-RBP (RBP+retinol) is bound to TTR, forming the Retinol-RBP-TTR complex.

35
Q

What happens once retinyl esters enter the hepatocytes?

A

They will be hydrolyzed into RETINOL form (reversible)

36
Q

How does vitamin A circulate in the blood, once released from the liver?

A

Retinol-RBP-TTR Complex

Retinol in the liver, will bind with RBP, leave the liver, and will bind to TTR found in plasma

37
Q

What is the process for storing retinol in the liver, if it’s not needed?

A

Retinol will be taken up by the stellate cells.

It will be converted to retinyl esters + small lipid droplets, and stored.

*THIS IS REVERSIBLE, so if vitamin A is needed, it can be converted back to retinol, bind with RBP and circulate in the plasma as the “sandwich complex.”

38
Q

What form of vitamin A is required for the visual cycle? Where must it be transported to?

A

RETINAL form

To the retina

39
Q

Rod Cells

A

helps our eyes adjust to the light;

40
Q

Cone Cells

A

used for color vision, such as red, blue, and green

41
Q

(T/F) Retinal needs to be transported to retina, so it can be used by the rod and cone cells to be utilized in the visual cycle?

A

TRUE

42
Q

Where does the visual cycle take place?

A

Between the pigment epithelium and the tips of the rod cells.

43
Q

How is vitamin A delivered for use in the visual cycle?

A

Retinol-RBP-TTR will be delivered via the capillaries

44
Q

Explain how rhodopsin is formed?

A

In the visual cycle, 11-cis-retinal is going to complex with OPSIN, forming rhodopsin.

45
Q

What is rhodopsin?

A

The pigment that is in rod cells; the visual pigment.

46
Q

Explains what happens when rhodopsin is hit by light?

A

When rhodopsin is hit by light (or bleached), it will convert the 11-cis-retinal —> all-trans-retinal, which splits off the opsin.

47
Q

Where is the rod cell located?

A

In the retina

48
Q

What is the importance of the rhodopsin getting “bleached?”

A

It will send an electrical signal to the optic nerve, and this is how light is perceived. Remember, the optic nerve is part of the brain.

49
Q

What happens in a dim/dark environment, when a flash of light hits the retina?

A

1) 11-cis-retinal is isomerized to all-trans-retinal
2) Rhodopsin is cleaved into opsin and all-trans-retinal.

Remember: bleaching occurs as rhodopsin splits, and transformation of rhodopsin leads to an electrical signal to the optic nerve and light is perceived!!

50
Q

What needs to happen for vision to be regained in the dark?

A

All-trans-retinal needs to be converted back to 11-cis-retinal, which binds opsin to reform rhodopsin.

*Remember: 11-cis-retinal is formed back in the pigment epithelial cells. So it has been regenerated it can move back into the rod cell and the cycle continues.

51
Q

RBP

A

Retinol Binding Protein

52
Q

Which form of Vitamin A acts like a hormone?

A

Retinoic Acid

53
Q

What the functions of retinoic acid?

A

Acts like a hormone:

-Cell differentiation
-Growth
-Reproduction
-Immune function

54
Q

Which forms of vitamin A are involved in gene expression?

A

all-trans retinoic acid, and
9-cis retinoic acid

55
Q

CRABP

A

Cellular Retinoid-Binding Protein

Binds to retinoic acid and acts as a transporter for the retinoic acid into the cell nucleus.

NHM 561 (21 decks)

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