Urea Cycle + Disorders Flashcards
What is the urea cycle, how does it begin and end
The urea cycle is the primary means through which cells, particularly hepatic cells, detoxify ammonia by converting it to urea.
The urea cycle begins and ends with ornithine, thus the term cycle.
How does ammonia enter the urea cycle
Ammonia enters the cycle in two ways:
1) as free ammonia prior to its incorporation into carbamoyl phosphate and
2) as the α-amino group of the amino acid, aspartic acid.
Reactions of the urea cycle (5)
Reaction 1: NH4+ enters the urea cycle by reacting with CO2 to form carbamoyl phosphate in a reaction catalyzed by carbamoyl phosphate synthetase I.
This reaction occurs in the mitochondrion and two ATPs are used in forming the high energy mixed anhydride bond of carbamoyl phosphate.
Carbamoyl phosphate is also used in the synthesis of pyrimidines, which occurs in the cytoplasm (discussed in MBLD&D, will become important later).
A second carbamoyl phosphate synthetase, CPSII, is found in the cytoplasm and functions in pyrimidine synthesis.
Reaction 2: Carbamoyl phosphate reacts with ornithine to form citrulline in a reaction catalyzed by ornithine transcarbamoylase. Citrulline is then transported out of the mitochondrion.
Reaction 3: The second nitrogen enters the pathway as aspartic acid, which reacts with citrulline to form argininosuccinate in a reaction catalyzed by argininosuccinate synthetase.
Reaction 4: Argininosuccinate is converted to arginine and fumarate by a reaction catalyzed by argininosuccinase.
Reaction 5: Arginase cleaves arginine to ornithine and urea.
Ornithine returns to mitochondria, urea to urine.
Urea cycle regulation
Substrate availability: The enzymes of the urea cycle are underutilized under most conditions and their level of activity is dictated by substrate availability.
Enzyme induction: Conditions that stimulate protein catabolism, high protein diet or prolonged starvation, cause an increase in expression of enzymes of the urea cycle.
Allosteric regulation: Carbamoyl phosphate synthetase I has an almost complete dependence on the allosteric activator, N-acetylglutamate (NAG).
Significance of N-acetylglutamate (NAG).
N-acetylglutamate is synthesized from glutamate and acetyl CoA in a reaction catalyzed by the enzyme, NAG synthase
NAG synthase has a high Km for glutamate (reservoir for nitrogen atoms under protein catabolic conditions) and so will be most active under conditions where there is a need for nitrogen detoxification.
NAG synthase is also stimulated by arginine, which indicates that it is most active when the urea cycle is operating at high efficiency to detoxify ammonia.
The use of acetyl-CoA as a second substrate for NAG synthase reflects the integration of protein and fat catabolism under fasting conditions where there would be a significant need for ammonia detoxification.
Which substrate of NAG synthase is an indicator that protein catabolism is high, signaling a need for increased capacity of the urea cycle?
Glutamin acid
What dictates amount of ammonia present in urea cycle disorders
he amount of ammonia accumulating in patients depends on where within the cycle the enzyme defect lies.
Features of CPS1 deficiency
Patients with a CPS1 deficiency fail to begin the cycle and as a consequence fail to consume ammonia in forming carbamoyl phosphate.
Without carbamoyl phosphate, citrulline levels fall affecting the argininosuccinate synthase reaction where the second nitrogen entering at the level of aspartic acid fails to occur, further reducing the amount of nitrogen being detoxified through conversion to urea.
Features of OTCase deficiency
Ammonia levels are also extremely high in patients with OTCase deficiency because elevated levels of carbamoyl phosphate will ultimately inhibit CPS1 interfering with the amount of ammonia entering the cycle.
Features of argininosuccinate synthase (citrullinemia) deficiency
The amount of ammonia accumulating in patients with a deficiency in argininosuccinate synthase (citrullinemia) is reduced compared to CPS1 and OTCase deficiency, because some ammonia is being detoxified in forming citrulline.
Citrulline accumulating in these patients is lost in the urine, taking with it some of the detoxified ammonia.
argininosuccinase deficiency fatures
Lower levels of ammonia
With this enzyme deficiency, nitrogens in the form of ammonia and aspartic acid both end up in the intermediate argininosuccinate, which can be lost in the urine (but not as efficiently as urea), taking 2 equivalents of nitrogen along with it.
What makes arginase deficiency unique
Hyperammonemia is relatively infrequent, unless under conditions of stress or with a high protein diet.
This make sense because the arginine accumulating under these conditions contains two nitrogens that were on their way to detoxification as urea.
Since a considerable fraction of arginine accumulating in this condition is lost in the urine, there is less nitrogen accumulating as ammonia.
Patients with an arginase deficiency have a delayed onset of symptoms (~ 3-years of age) compared with patients with other urea cycle defects.
Patients with an arginase deficiency have a neurological complication that is not seen in other urea cycle defects (spastic diplegia/paraplegia) indicating that elevated levels of arginine disrupt neurological function in a manner distinct from the neurotoxicity of ammonia.
How can plasma amino acid levels be used to determine urea cycle disorders
In three urea cycle disorders the plasma amino acid accumulating is the intermediate in the urea cycle immediately preceding the enzyme affected (citrulline accumulating in a patient with an argininosuccinate synthase deficiency, argininosuccinate in a patient with a argininosuccinase deficiency, and arginine with an arginase deficiency).
The situation is a little more complicated with the early urea cycle defects where glutamic acid, alanine and lysine accumulate.
If the urea cycle is blocked early, ammonia increases inhibiting glutamate dehydrogenase thereby causing an increase in glutamic acid levels.
Early urea cycle defects also block the incorporation of nitrogen into the cycle at the level of aspartic acid, which would also lead to an increase in glutamic acid levels as the equilibrium of aspartate transaminase would be shifted toward glutamic acid production.
As glutamic acid levels rise, they will shift the equilibrium of the alanine transaminase reaction away from glutamic acid toward alanine, thus increasing alanine levels as well.
two causes of orotic aciduria in humans
One is caused by a defect in pyrimidine synthesis and the other an OTCase deficiency.
Orotic acid in urine can differentiate defects due to deficiencies in CPS1 versus OTCase.
Mechanism of Orotic aciduria as a result of a defect in pyrimidine synthesis
Orotic acid is an intermediate in the pathway used to synthesize the pyrimidine nucleotides UMP and CMP.
A deficiency in the multifunctional enzyme UMP synthase (shown with the red X in the figure below) leads to elevated levels of orotic acid and reduced amounts of UMP and CMP.
Orotic aciduria caused by a deficiency of UMP synthase can be treated with uridine, which supplies the pyrimidine nucleotides, which in turn, feedback inhibit early steps in the pathway reducing orotic acid levels
Mechanism of Orotic aciduria as a result of an OTCase deficiency
An OTCase deficiency leads to an accumulation of carbamoyl phosphate within mitochondria
Increased amounts of carbamoyl phosphate exit the mitochondria and stimulate pyrimidine synthesis increasing orotic acids levels, a fraction of which ends up in the urine causing orotic aciduria.
Process of Eliminating excess nitrogen from the body by means other than the urea cycle
Ammonul is a mixture of sodium phenylacetate and sodium benzoate, which is used to reduce the nitrogen load in patients with urea cycle defects.
Phenylacetate reacts with glutamine to form phenylacetyl glutamine, which is eliminated from the body in the urine
To replenish glutamine pools, glutamine synthase uses ammonia and glutamic acid to form glutamine.
Benzoate reacts with glycine to form hippuric acid, which is subject to renal elimination.
Glycine is a non-essential amino acid that is replenished from serine, which itself is derived from the glycolytic intermediate 3-phosphoglycerate
Importantly replenishing glycine levels through the pathway outlined in Fig. 10 consumes glutamic acid, which is a primary source of ammonia via glutamate dehydrogenase.
