L25: Detoxification of Ammonia: Urea & Glutamine Synthesis

Question 1

LO1: List the metabolic functions of glutamine and indicate organs where they are most important

Answer 1

1. precursor form of nitrogen for pathways, especially purine and pyrimidine synthesis
2. nontoxic transporter of ammonium ion from extrahepatic tissues (esp. imp. in brain) to liver where it can be converted to urea
3. major fuel for enterocytes (gut) and is partially metabolized to provide precursors for other AAs
4. major fuel for macrophages and lymphocytes for immune system
5. maintains acid-base balance in kidney (glutaminase provides sink for protons by hydrolyzing the amide group in the side chain of glutamine)

Question 2

LO2: Describe the reaction by which glutamine is synthesized. Where does it occur and what is the physiologic significance of this reaction?

Answer 2

alpha-ketoglutarate—–transaminase—->glutamate +NH4+——glutamine synthethase—->glutamine

• occurs in extra-hepatic tissues (esp. brain and muscle)
• synthesis of glutamine allows for conversion of NH4+ into a nontoxic organic form
• imp. in brain as NH4+ is very toxic here, and in muscle because protein turnover is high here and leads to lots of NH4+

Question 3

LO3: Describe how the kinetic properties and hepatic localization of CPS-I and glutamine synthetase contribute to efficient detoxification of ammonia by the liver

Answer 3

CPS-I: catalyzes first and committing step of urea cycle

• kinetic properties: high Km/low affinity for NH4+, so triggered by high levels of ammonia (glutaminase) to catalyze urea cycle
• hepatic localization: mitochondria of periportal hepatocytes

GLUTATMINE SYNTHETHASE

• kinetic properties: has a low Km for NH4+, meaning high affinity, so it is a good scavenger for NH4+
• hepatic localization: present in all tissues but especially in brain and muscle (see LO1)
• localized in liver in perivenous hepatocytes
• glutamine can be degraded in liver (periportal hepatocytes-glutaminase) to reform NH4+ and NH4+ can be converted to urea

Question 4

LO4: Identify the immediate precursors for the N, C, and O atoms in urea

Answer 4

N (amino group): 1 comes from aspartate and 1 comes from ammonium ion

C=O (carbonyl group): comes from bicarbonate

Question 5

LO5: List the properties of urea that make it a good physiologic choice as a molecule for disposal of waste nitrogen

Answer 5

• simple/low energy to make
• small, easily passes through membranes
• polar, so soluble in blood
• non-toxic, so good for transport

Question 6

LO6: Describe the relationship between ureagenesis and gluconeogenesis

Answer 6

ALANINE-GLUCOSE

Ureagenesis: uses N to make urea which can be excreted

Gluconeogenesis: breaks down AAs to make glucose which generates N

Alanine from skeletal muscle is used to transport ammonia for ureagenesis to liver, and is then used to regenerate pyruvate in liver, which can be used for gluconeogenesis

Question 7

LO7: Understand the energy cost for synthesizing each molecule of urea

Answer 7

3 ATP (from FA oxidation), 4 high energy bonds broken

1. CPS-I uses 2 ATP to convert ammonia and bicarb to carbamyl-p (left with 2ADP)
2. Transfer of citrulline to argino-succinate by ASS uses 1ATP, but breaks 2 bonds (left with 1AMP)

Question 8

LO8: Compare and contrast the mechanisms for short and long term regulation of the urea cycle

Answer 8

SHORT TERM REGULATION OF UREA CYCLE
-synthesis by CPSI of carbomyl phosphate (+ by NAG, which is synthesized inside mt from glutamate and acetyl-CoA, whose synthesis is + by arginine)

LONG TERM REGULATION OF UREA CYCLE

• transcriptional changes of urea synthesis enzymes, which change in response to dietary protein levels
• high protein diet and starvation increase enzyme activity (due to increased protein breakdown)
• protein free diet decreases enzyme activity

Question 9

LO9: Identify inherited defects in specific urea cycle enzymes by changes in levels of various urea cycle intermediates in plasma or urine

Answer 9

ALL UREA CYCLE DEFECTS RESULT IN HYPERAMMONEMIA
-if respiratory distress occurs prior to 24hours after birth, then this is due to a transient hyperammonemia and not an inborn error of metabolism

Acidosis: methylmalonate or propionate metabolism defect

Acidosis absent: Ureagenesis defect

Absent citrulline: CPSI deficiency if urinary orotate is also low; OTCase deficiency if urinary orotate is high

10-300uM of citrulline: ASL deficiency (will also see arginosuccinate and anhydrides in plasma)

> 1000uM of citrulline: ASS deficiency (will also see low arginine; increase in citrulline because it gets backed up)

Question 10

L10: Describe the basis for treatment of urea cycle disorders with benzoate and phenylacetate and the relative effectiveness of each

Answer 10

SODIUM BENZOATE TREATMENT

• given to promote alternate pathways that eliminate nitrogen
• benzoate=precursor for hippuric acid
• AA Nitrogen in form of glycine added to hippuric acid from AA pool to be excreted in urine

SODIUM PHENYLACETATE TREATMENT

• same principle as benzoate
• phenylacetate=precursor for phenylacetylglutamine
• AA Nitrogen in form of glutamine added to penylacetylglutamine from AA pool to be excreted in urine

-both work because glycine and glutamine are in equilibrium with N atoms in the free amino acid pool, so can act as sinks