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