Vitamin A: Introduction

00:02 In this lecture, I’m going to describe vitamins D and A, two of the fat-soluble vitamins and some of the very different functions that each one has.

00:11 Now, vitamins D and A, as I said, are fat-soluble vitamins.

00:15 They are important in the case of vitamin A for vision.

00:18 In the case of vitamin D, for calcium metabolism.

00:20 But that’s not the only thing that these vitamins do.

00:24 Vitamin A is also important for gene expression and differentiation that occurs within organism during development.

00:30 Vitamin D is also important for controlling gene expression and essential for a healthy immune system.

00:36 As we will also see, it has some anticancer properties as well.

00:40 Now, one of the things to be careful about with vitamins A and D is that because they’re fat soluble, excessive amounts of either one of them could be harmful because they get stored in fat tissue and can be released over a long period of time.

00:52 So these are two vitamins you don’t want to take in excess amounts.

00:56 Vitamin A, as I said, is fat soluble.

00:59 It is toxic at high doses and it’s actually possible for a person to overdose on eating too much liver from certain organisms like polar bears, so stay away from polar bears.

01:08 Vitamin A is stored in the liver and it occurs in three forms in the body.

01:14 The alcohol form is known as retinol and you can see it outlined on the screen.

01:19 On the far right of the molecule, there’s an alcohol group and that’s what gives it the –ol ending of its name.

01:25 Retinal is a related compound that differs only in containing an aldehyde group at the end of its structure.

01:31 And finally, retinoic acid has a carboxyl group at its end.

01:35 Now these three different molecules have three very different functions within our body.

01:41 Now, retinol is the storage form of vitamin A.

01:45 To store vitamin A, retinol is esterified to a fatty acid as you can see on the structure on the top.

01:51 Now, vitamin A comes in a variety of isomeric forms.

01:54 And the form that you see on the top for retinol is known as the all-trans form, that is all the double bonds are in the trans configuration.

02:01 Those double bonds can isomerize in the presence of light, meaning that light can actually physically change their structure.

02:09 The summarization of retinal is what gives rise to vision as we shall see.

02:13 And one of those isomers you can see in this structure which is the 11-cis retinal isomer.

02:19 And it’s this isomer that is stored within our eyes.

02:22 Retinoic acid is important for cell differentiation.

02:25 Without retinoic acid, we won’t form into the organisms that we do.

02:30 Now, vitamin A is produced in the body using beta carotene as a precursor.

02:34 And beta carotene is shown in the figure at the bottom.

02:37 Basically, if you cut beta carotene in half, you will get one of the forms of vitamin A that you see on the screen here.

02:45 Now, 11-cis retinal, as I said, is important for vision.

02:49 So I need to describe to you that process by which this occurs.

02:52 First, 11-cis retinal is bound to a protein called opsin.

02:56 And it’s through protein called opsin that retinal provides us with vision as we shall see.

03:02 The absorption of a photon of light by the 11-cis form of retinal causes the 11-cis form to flip back to the trans configuration.

03:12 So we can see this flipping process occurring here where we see it flipping on the top to the bottom or from the bottom back up to the top.

03:19 and this happens very readily with this form of vitamin A.

03:23 It’s the change in structure, the change in form, that actually provides the very first signal in our eyes that light has been detected.

03:32 So before I talk about the biochemistry of vision, I’ll just say a little bit about the actual physiology and the cell structure that gives rise to vision.

03:40 So vision happens as a result of light detection that occurs in specialized cells in our eye known as retina cells.

03:47 The opsin that I described earlier is the protein that actually holds vitamin A containing compound, the retinal, that allows us to have vision.

03:55 Now, notice that retina and retinal are pretty similar in terms of name, but they’re very different things.

04:02 We have two types of cells in our eye that detect light.

04:05 The most abundant cells that we have are known as rod cell.

04:08 And they provide very basic functions of light detection.

04:11 They don’t provide for differences in color, but rather simply detection of a photon of light.

04:18 The photo pigment that they contain, the opsin, contain links to the retinal, is known as rhodopsin.

04:24 The other cells that we have in our eyes are known as cone cells.

04:27 And they actually provide the color detection that we see when we see a well lit room.

04:32 They also have retinal linked to an opsin.

04:35 But the opsin there is a little bit different and so we call those photopigments, photopsins.

04:41 There are three types of cone cells.

04:43 One type specialized for the absorption of red wavelengths of light.

04:47 One type of cone cells specialized for the detection of green type of light and one specializes for blue types of light.

04:55 Now, to give you an idea of abundance, there are about a hundred and twenty million rod cells per eye and about 6 million cone cells per eye.

05:02 So eyes have pretty good resolution in terms of being able to detect small amounts of light.

05:10 The rod cells can detect.

05:11 They’re so sensitive that they can detect individual photons of light.

05:15 Now, that detection comes at a price.

05:17 They can’t tell the color of that light, but they can tell whether or not light impinged upon them.

05:23 The opsin as I said earlier contains the 11-cis retinal and in that form, it’s known as rhodopsin.

05:29 The retinal in rhodopsin isomerizes in response to the light and that isomerization causes the retinal to change form.

05:37 So instead of being bent as we saw earlier in the structure, it straightens out.

05:42 Well, that straightening out of the retinal that happens in the presence of light causes the rhodopsin that contains it to also change its structure.

05:51 Now, one of the things we’ve seen in other lectures that I’ve given here relates to small modifications in protein structure.

05:58 Small modifications in protein structure can have very big effects on what the protein actually does.

06:03 And that’s very much the case here with rhodopsin.

06:06 So these retina cells that I’m describing to you are very sensitive and that sensitivity allows them to very easily send signals to the brain that they detected something.

06:17 Those signals happen as a result of chemical signals that arise in the nerve cells.

06:22 Now, this chemical process is kind of complicated.

06:24 So I’m going to take you through it slowly.

06:27 The unstimulated optic nerve cells are what we call unpolarized, meaning that they have an even distribution of ions on the outside and on the inside of the cell.

06:37 This is very different from other nerve cells because they start out in a polarized fashion.

06:43 Light stimulation that happens on the nerve cells of the eye, however, caused hyperpolarization.

06:50 This is exactly the opposite of other nerve cells.

06:53 Other nerve cells start out hyperpolarized and stimulation causes them to become unpolarized.

06:59 So we see the reverse of the process that’s happening with nerve cells in our eyes.