In the lectures on amino acid metabolism,
I've talked about how amines are moved
from one molecule to another, and how ammonia
is a toxic byproduct of their metabolism.
With this lecture, I'll talk
about how those amines and
how that ammonia are combined
and prepared for excretion.
Nitrogen in the body, as we have
seen, comes in various forms.
Balance is absolutely critical.
Nitrogen is an
but it’s also one that two
high levels can be toxic.
The body must be able to handle the nitrogen
arising from amino acid metabolism.
But nitrogen is also needed for
the synthesis of other molecules.
This includes the nucleotides, ATP, GTP,
CTP, UTP, dATP, dCTP, dGTP and dTTP.
Nitrogen is also needed for making
of non-protein amino acids.
These include ornithine, citrulline
and sarcosine as shown in the screen.
And there are yet other
nitrogen containing compounds
that are necessary for
synthesis in the body.
This includes choline, various
vitamins and carnitine.
Now, the nitrogen cycle is a very
important cycle in our environment.
Nitrogen is a -- the nitrogen in
our atmosphere must be reduced.
And there are nitrifying organisms at the
very deepest parts of our soil that grab
that nitrogen and convert it into the amines
that are useful for higher organisms.
That over all process
is shown on the slide.
Now, nitrogen must be excreted if the
balance is to be properly maintained.
As I've noted, amino acids through
transamination make nitrogen mobile.
It's very easy to move nitrogen from one
amino acid to a keto acid and vice versa.
The toxicity of ammonia means
that the nitrogen that's produced
as a byproduct of amino acid
metabolism must be handled properly.
In the excretion process,
there are various
strategies that are employed
by different organisms.
Organisms for example that are
ammonotelic - excrete ammonia.
This includes fish and this is why you
have to periodically clean your aquarium.
Uricotelic organisms excrete uric acid and these include birds.
include human beings.
These excrete urea and it also includes
most vertebrates and a few invertebrates.
Now, when we look at the structure of
these molecules, we see of course that
ammonia only contains one nitrogen but has
the advantage that it's water soluble.
It has the disadvantage of
course that it's toxic.
Uric acid is a very good way to
get rid of a lot of nitrogen.
It has four nitrogens within
it but suffers from the
disadvantage that it is
not very water soluble.
Urea is the sort of the perfect
compromise of the two.
It contains two nitrogens for excretion
and it's very water soluble.
The ammonia that's produced by amino acid
catabolism is used in the urea cycle.
And the uric acid that's
produced by purine catabolism
is a byproduct of the
breakdown of nucleotides.
And finally, urea is
produced by the urea cycle.