Oxygen free radicals & cellular injury – causes, symptoms & pathology

So what exactly is a free radical? It’s
a chemical species with an unpaired electron in its outer orbit. One example, and probably
actually the most well-known example, is oxygen. Normally oxygen’s got the 4 pairs of electrons
here, but in a free-radical situation, it might GAIN an extra electron, so now you’ve
got this one unpaired electron and we call this guy a free radical and it’s what causes
all the trouble and potentially can cause cellular injury. Free radicals are generated both normally
in physiologic conditions and abnormally in pathological conditions. Physiologically,
so we’re talkin’ like in normal conditions, it can happen during oxidative phosphorylation,
which we know is this super important process in our cells to make ATP. During this process, you’ve got this molecule
called cytochrome c oxidase, or sometimes it’s known as complex IV. This molecule
transfers electrons to oxygen, and oxygen gladly accepts those electrons, and this in
turn causes the mitochondrial matrix to pump protons out. And this creates our proton gradient
and drives the production of ATP. So normally, for oxygen to be happy it accepts
4 electrons and becomes water. But what if it doesn’t receive all 4? Well that, my
friends, is when we get free radicals. Okay so if Oxygen grabs one electron, it becomes
SUPERoxide, so O2 with a little dot for its extra electron. If it gets two electrons,
it becomes hydrogen peroxide, and then 3 electrons, it’s hydroxyl radical, then finally if you
get all four electrons you get water. So 1, 2, or 3 electrons is called a partial reduction
of oxygen, so oxygen isn’t being reduced all the way to water, because remember that
reduction is a gain of electrons, so in this case it’s only partial. And remember that
this happens physiologically, in like normal conditions. Also though, you can generate free radicals
in pathological conditions too, so not normal conditions. One way is through ionizing radiation, and
this usually generates a hydroxyl radical. And this’ll happen when the radiation sort
of comes in and hits water in your tissue, so it hits the h2o right? And it knocks off
an electron, and if so if you go backwards here from water, you’ll end up at the hydroxyl
radical. And these hydroxyl free radicals are generally the most reactive, and so therefore
probably the most damaging to your cells. Inflammation is another way though that you
can get free radical generation. When you get an infection and neutrophils come by to
fight that infection, there are two mechanisms they use to kill whatever microbe there is,
oxygen dependent and independent. Oxygen dependent means you need oxygen right? So we can probably
guess that oxygen’s involved! Usually this starts with what’s called an oxidative burst,
where an enzyme called NADPH oxidase takes your oxygen and converts it to superoxide
(O2-), and then it goes on to be eventually converted into bleach, or HOCL. But mainly
our culprit for producing free radicals is gonna be this first step where NADPH converts
oxygen to superoxide. Another way this can happen too is through
contact with metals, and we’re usually talking about metals in the context of the body, so
like copper or iron, right? Where they’re usually bound to something? One example something
are called tranferrins, these proteins bind iron and sort of help you control the amount
of iron in your blood. Why do we wanna make sure it’s controlled? Well because if it’s
not bound, it can generate free radicals. A reaction called the fenton reaction allows
iron to generate a free radical, usually the hydroxyl free radical, our most dangerous
guy! So let’s think about it in the context of
some disease, like hemochromotosis. Where iron builds up, and you get serious tissue
damage, like cirrhosis in the liver. The primary mechanism behind this is a buildup of free
radicals! Similarly, in wilson’s disease, you there’s an excess of free copper, so
copper that’s not bound. You also end up getting this tissue damage from free radicals. Finally, there’re drugs and chemicals that
can produce free radicals. For example a drug like acetaminophen, which, like many other
drugs goes to the liver to be metabolized. In the liver, the P450 system takes care of
it, which is like this group of really important drug-metabolizing enzymes. This metabolizing
though, can generate free radicals, and so when high doses of acetaminophen are taken,
it can cause massive death of tissue in the liver, and this is mainly from free radical
damage. So how do free radicals injure cells? Well
one big way is called lipid peroxidation. Meaning that they can sort of steal an electron
from the lipids of cell membranes. Remember that they have this unpaired electron, so
they want another electron to be its pair, so what can happen is it takes an electron
from a lipid in the cell membrane, which leaves that lipid with an unpaired electron, and
and then the same thing happens and it sort of propogates, which as you could imagine,
hurts the cell membrane and damages the cell as a whole. They can also oxidize both proteins
and DNA inside the cell as well. Oxidation of proteins can obviously hurt the cell depending
on the protein’s function, but oxidation of DNA is serious, and is a super big player
in oncogenesis, or causing cancers, since cancers are formed by mutations of DNA. So
if these free radicals oxidize DNA and introduce mutations, all the sudden your risk for developing
a cancer goes up. Okay so since this can happen in both pathologic
AND physiologic settings, since our bodies are so smart, it’s totally reasonable that
we have ways of getting rid of them right? Yes! It is! The first defense against these
oxidants are uhhh antioxidants! That makes sense, right? Because oxidants, like free
radicals, can take electrons, so antioxidants, like vitamin A and vitamin C, vitamin E, which
can all eliminate free radicals by donating electrons. Another way that we can get rid of them, which
we touched on, are metal carrier proteins, like transferrins and ceruloplasmin, which
respectively bind or carry iron and copper in the blood. For example, transferrin carries
and delivers it to the liver and macrophages. And then when bound in macrophages or the
liver by a molecule called ferritin, it’s sort of like hidden away and so it’s not
able to generate free radicals. And finally, another way we can get rid of
free radicals is by enzymes, and there are three super important players in the free
radical game. So remember that in between oxygen and water, you’ve got superoxide
with one electron, then hydrogen peroxide with 2, and finally hydroxyl with 3, and then
water with four, right, okay. Now, our three enzymes are each going to have the job of
focusing on one of these free radicals. Superoxide is taken care of by superoxide dismutase,
that ones easy; an enzyme called catalase takes hydrogen peroxide, and then hydroxyl
free radical is taken by glutathione peroxidase, which is less intuitive, because maybe you’d
think that that one should be for hydrogen peroxide, but it’s not. Okay let’s quick go over two clinical examples
of injury due to free radicals. The first is from a chemical called carbon
tetrachloride, so one carbon, then four chlorides – and this guys actually used in the dry cleaning
industry. If it gets into the blood, it’s converted to trichloromethyl radical, or CCl3
which is a free radical, and this happens in the P450 system of the liver. Now that
it’s a free radical, it starts wreaking havoc on the hepatocytes of your liver, right?
It can start damaging proteins, DNA, and cell membranes. In the early stages, this damage
is actually reversible, and one way you can tell is by looking for cellular swelling,
so the cells are actually swelling. This causes the rough endoplasmic reticulum of the cell
to also swell. And remember how on your Rough endoplasmic reticulum, you’ve got all your
ribosomes right? Which help make proteins. So when it swells they pop off and your protein
synthesis goes down. And what does our liver do again? OH right, right it gathers up fat
and cholesterol from the diet right? and repackages it and sends it back on it’s way. This repackaging
process is done by our good friends the apolipoproteins. apolipoPROTEINS. Proteins that help receive,
pack, and send back out the fats and cholesterol. So if CCl4 or rather the trichloromethyl radical
damages the liver cell, and causes swelling and loss of ribosomes, then you’ll decreased
production of proteins…and decreases our friends the apolipoproteins. Now all the sudden
you’ve got these fats coming into the liver but not being repackaged and sent back out…and
lo and behold, the fat doesn’t escape and you get this fatty change in the liver. Check
out this histology of some hepatocytes. These circular spaces represent the accumulation
of fat, or fatty liver. So this fat buildup in the liver is ultimately caused by free
radical damage. Alright, another free radical injury example
is Reperfusion injury. So let’s say you cut the blood supply to some organ, like your
heart, which might lead to a myocardial infarction. If tissue starts to die, the cell membranes
will start to go and you’ll get enzymes leaking out into the blood. So if for some
reason, now blood flow is restored to that organ, you’ve got this oxygenated blood
coming back to the organ with inflammatory cells. These inflammatory cells reacting with
the dead tissue IN the presence of oxygen, and since it’s in the presence of oxygen,
you can start generating free radicals, which can damage the organ’s cells even more.
So if a patient has a myocardial infarction, their cardiac cells start to die and their
cardiac enzymes will go up, if the artery is then opened and blood flow returns, the
cardiac enzymes might KEEP rising because now you have oxygenated blood combined wiht
inflammatory cells, which continues to injure the myocardium. And this is called reperfusion
injury! So cardiac enzymes just keep rising even after the blood supply returns to the
region, all because of free radical generation.