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  #1  
Old 10-17-2015
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Default Urea cycle

In CPS 1 or NAG synthetase deficiency, why doesn't the body convert excess ammonia into glutamine and then make carbomyl phosphate using CPS 2?
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Old 10-17-2015
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Default Nice question!

I am not entirely sure of this, but well, my two cents.

CPS-I is a mitochondrial enzyme, while CPS-II is a cytoplasmic enzyme. I am guessing CPS doesn't easily move from cytoplasm to mitochondria. Specially when considering that that a relatively huge amount of CPS has to accumulate in mitochondria, before it *leaks* into the cytoplasm (in case of Ornithine transcarbamoylase deficiency).

So the CPS you make from CPS-II will go into pyrimidine synthesis, rather than urea.

Again, this is conjecture/deduction 50/50
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Old 10-17-2015
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Are you sure about the part where a huge amount of carbomyl phosphate has to accumulate in mitochondria before it moves into the cytoplasm? I was under the impression that it moved freely. But then your explanation does make sense
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Quote:
Originally Posted by datson View Post
Are you sure about the part where a huge amount of carbomyl phosphate has to accumulate in mitochondria before it moves into the cytoplasm? I was under the impression that it moved freely. But then your explanation does make sense
I assumed it is so. if CPS could easily diffuse out of the mito, even a normal urea cycle wud be giving rise to inc pyrimidine synthesis/orotic aciduria. or u can argue that OTC has a really high Km, which i have no idea about.

Here is something else, OTC is only found in hepato mitochondira, CPS2 working anywhere else in the body will over produce CPS which can not be metabolized in that cell to urea.
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Old 10-17-2015
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No that doesn't make sense.
There is no excess carbomyl phosphate being formed in CPS 1 deficiency because there is no orotic acuduria.
Ok I just thought of this. Maybe CPS 2 is suppressed due to reduced ATP due to inhibition of TCA cycle by hyperammonemia.
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Quote:
Originally Posted by datson View Post
No that doesn't make sense.
There is no excess carbomyl phosphate being formed in CPS 1 deficiency because there is no orotic acuduria.
No, u misunderstood. I wasn't talking of a deficient state. I was implying that in a normal individual, if CPS easily diffused out of mitochondria, a little (at least) wud be expected to enter cytoplasm, with every cycle of CPS synthesis. And i don't think that happens.


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Originally Posted by datson View Post
Ok I just thought of this. Maybe CPS 2 is suppressed due to reduced ATP due to inhibition of TCA cycle by hyperammonemia.
Actually, I didn't know that.
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K got it.
I guess its a combination of these factors.
Awesome brain-storming session!
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Yeah, this was pretty good!
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