Other Lipids and Their Function – Lipids

by Kevin Ahern, PhD

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    00:00 is pregnenolone.

    00:02 Now there are other lipids that are important, and they're important in regulating body processes as well. These are derived from other existing lipids, they include the bile acids and salts, they include waxes, prostaglandins, thromboxanes and leukotrienes. Now if we look first of all at the bile acids, we can see that the bile acids sure look a lot like cholesterol and in fact the bile acids are derived from cholesterol, just like the steroid hormones are. Different modifications and in fact the different modifications are significant in the sense that the modifications make these molecules much more polar and much more ionizable.

    00:46 So we can see deoxycholic acid for example on the left has carboxyl group located on them, they can ionize, and we see taurocholic acid has a sulfate in it that can readily ionize as well. Now when we have molecules that have nonpolar portions and polar portions, we remember of course these molecules are called amphiphilic. But amphiphilic molecules are also notable in that they can act like detergents. Fatty acids you may recall are amphiphilic and we use them for soaps. Detergents are important for helping to dissolve lipid compounds, that's why we wash our hands with soap, to get rid of oils and greases and so forth because they can solubilize that water. The fat that we get in our diet is important to be solubilized as well and the way that they gets solubilized in our digestive system is by being wrapped up with the bile acids that you see on the screen.

    01:42 Waxes are very nonpolar compounds that our inner ears for example, or you can see an example on the screen of beeswax, they're very nonpolar because they have an ester between a fatty acid and a long chain alcohol. Now that creates a molecule like cetyl-palmitate that you can see on the screen here and the cetyl-palmitate has virtually nothing that will associate with water and of course anybody who's ever worked with wax knows in fact that wax will not dissolve in water.

    02:13 Prostaglandins are very interesting and unusual class of compounds. They're made not from cholesterol, but rather made from arachidonic acid, the long polyunsaturated fatty acid.

    02:26 Arachidonic is depicted here in the green portion of the screen at the very top. Two different ways of writing the same molecule either as a bent molecule or as a linear.

    02:38 I happen to like the bent molecule on the left because it helps us to see what happens to arachidonic acid when it becomes metabolized to make the prostaglandins. Now there is an enzyme called cyclooxygenase that will convert arachidonic acid into prostaglandins.

    02:54 That cyclooxygenase creates, as you can see in the prostaglandins, a five membered ring in the left side of each of the molecules that are there. That five membered ring did not exist within the arachidonic acid and further, you can see that it has added some oxygens or hydroxyl groups to that five member ring. It's because of these two processes that these enzymes are called cyclooxygenases. Now prostaglandins have some very important and in some cases, contradictory effects on the body. They act like hormones but they're not hormones because they're very, very unstable. They usually act very close to the place where they're synthesized, but the effects that they can have on the body are remarkable.

    03:40 They can cause vasodilation, they can cause vasoconstriction, they can cause platelet aggregation, they can cause uterine contractions and they can cause pain. Now all of these arise from different prostaglandins, there are many, many prostaglandins that are made in the body. But one of the things that's done to prevent for example pain, is to prevent the synthesis of prostaglandins and the way that you prevent the synthesis of prostaglandins is by inhibiting the cyclooxygenase. Cyclooxygenases are inhibited by compounds like aspirin, so aspirin is a painkiller because it's inhibiting the production of prostaglandins, one of the things that it does is creates pain. Another thing that aspirin does in preventing the synthesis of prostaglandins is it also prevents the synthesis of things that cause platelet aggregation. Platelet aggregation of course, is linked to blood clotting. So when people take something to help them reduce the incidence of blood clots, sometimes the thing that they're taking is actually aspirin.

    04:47 Now thromboxanes are related to prostaglandins, in fact they're derived from prostaglandins and they have a structure that looks sort of like the prostaglandins, although they have been quite a bit modified from the prostaglandins. Thromboxanes, they have roles in vasoconstriction like the prostaglandins do and they also have roles in blood clotting. So taking the aspirin to reduce the clotting incidents as a result of prostaglandins has the dual effect of reducing the production of thromboxanes as well.

    05:18 Now leukotrienes are another important and interesting group of compounds that look like they're related to the prostaglandins, but in fact they're not made from the prostaglandins.

    05:29 Leukotrienes are made from arachadonic acid in a different pathway called the linear pathway.

    05:33 You can see some of the leukotrienes on the screen here. Leukotrienes mediate inflammatory responses in immune cells. Now inflammation is a very important part of the body's protective defense that sometimes can overwhelm things. Leukotrienes are involved in histamine production and they have a role in asthma. Inhibitors of the synthesis of leukotrienes are being investigated for their potential to stop asthma attacks.

    About the Lecture

    The lecture Other Lipids and Their Function – Lipids by Kevin Ahern, PhD is from the course Biochemistry: Basics.

    Included Quiz Questions

    1. They are derived from arachidonic acid
    2. They are precursors of eicosanoids
    3. They are broken down by cyclooxygenases
    4. They have synthesis stimulated by aspirin and NSAIDs
    1. Leukotrienes
    2. Prostaglandins
    3. Thromboxanes
    4. Arachidonic acid
    5. Taurocholic acid
    1. The prostaglandins are strong endocrine hormones produced by the pituitary gland
    2. Leukotrienes are synthesized from arachidonic acid via a linear pathway and plays an important role in inflammatory response and histamine production
    3. Prostaglandins act as powerful, locally acting vasodilators, and inhibit the aggregation of blood platelets
    4. The enzyme cyclooxygenase mediates the conversion of the arachidonic acid to prostaglandins
    5. Thromboxanes molecules play an important role in the vasoconstriction and blood clotting

    Author of lecture Other Lipids and Their Function – Lipids

     Kevin Ahern, PhD

    Kevin Ahern, PhD

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