Lecture 61

Question 1

urea cycle overview (aka ornithine cycle)

Answer 1

• cycle of biochemical reactions producing urea from ammonia (NH₃)
• first metabolic cycle discovered
• takes place primarily in the liver, and to a lesser extent in the kidney
• consists of 5 reactions (1&2 are mitochondrial, 3-5 are cytosolic)

pg 1556

Question 2

urea cycle steps

Answer 2

uhh see slide I guess

pg 1557

Question 3

rate-limiting regulated step of urea cycle

Answer 3

• enzyme: carbamoyl phosphate synthetase I (CPS-I)
• regulation:
• … by substrate availability (short term) → goal is to remove the toxic free ammonia, so contrasts feedback regulation
• … enzyme induction (long term)
• … allosteric regulation by N-acetylglutamate (NAG) → increases the affinity of CPS-I for ATP

1. bicarbonate provides carbon of urea
2. free ammonia provides one of the nitrogen atoms of urea
3. enzyme has an absolute requirement for N-acetyl-glutamate (NAG) which acts as an allosteric activator (stimulated by high levels of Arg)

pg 1558

Question 4

high protein diet

Answer 4

• stimulates urea cycle
• increases transcriptional synthesis of urea cycle enzymes

pg 1559

Question 5

step 2: ornithine transcarbamyolase (OTC)

Answer 5

• enzyme: ornithine transcarbomylase (OTC)
• in the mitochondria
• takes ornithine and carbomyl phosphate (high-energy molecule) and makes citrulline
• citrulline transported to cytosol
• deficiency in this enzyme is most common urea cycle defect (X-linked)

pg 1560

Question 6

ornithine and citrulline

Answer 6

• AA intermediates, NOT in polypeptide chains
• as citrulline is moved out of the mitochondria, ornithine is moved in

pg 1560

Question 7

steps 3-5 of urea cycle

Answer 7

• all steps in the cytosol
• second amino group added from Asp
• common intermediates with TCA cycle (malate, fumarate, oxaloacetate)
• enzyme: arginase → releases free urea which is transported into the bloodstream, filtered by the kidneys, and excreted in the urine
• following completion of the cycle, ornithine is regenerated and transported back into the mitochondria to be reused

pg 1561

Question 8

step 3: citrulline + aspartate to argininosuccinate

Answer 8

• utilizes argininosuccinate synthetase (needs ATP)
• amino group of aspartate provides one of the nitrogen atoms of urea

pg 1561

Question 9

step 4: argininosuccinate to L-arginine

Answer 9

• utilizes argininosuccinate lyase
• removes fumarate

pg 1561

Question 10

step 5: L-arginine to L-ornithine

Answer 10

• utilizes arginase-I which cleaves using water
• urea is removed from arginine

pg 1561

Question 11

serum urea nitrogen (BUN)

Answer 11

• normal value between 11-23 mg/dL
• determines how efficiently urea is produced
• if not working, increased ammonia in bloodstream, decreased urea in urine

pg 1561

Question 12

role of kidneys in nitrogen disposal

Answer 12

• removal of urea from circulation via filtration
• production of free ammonia via glutaminase reaction in the kidney tubular cells
• free ammonia released in urine as it picks up H+ (NH4+)
• release of H+ contributes to acid-base balance of the body by excreting protons
• ~10% of nitrogen in urine is free ammonia (NH4+) which gives the smell to urine

pg 1562

Question 13

deficiencies of urea cycle enzymes

Answer 13

• all present with high-blood NH3 (hyperammonemia → neurotoxicity)
• urea-cycle intermediates before the block accumulate leading to glutamine levels increased in the circulation
• prevents the α-ketoglutarate regeneration by removal of nitrogen from glutamine and free ammonia
• defects in any enzyme lead to elevated glutamine and ammonia levels in the blood
• the extent of the elevation depends on which enzyme is defective
• symptoms similar to drug/alcohol intoxication

pg 1564

Question 14

OTC deficiency

Answer 14

• most common inborn urea cycle disorder
• x-linked
• neonatal encephalopathy
• carbamoyl phosphate accumulates and floods pathway for pyrimidine biosynthesis; ornithine also accumulates
• excess orotic acid (intermediate) is excreted in urine; no ill effects, but indicative of problem in urea cycle

pg 1565

Question 15

treatments for urea cycle defects

Answer 15

• key: early diagnosis, aggressive treatment to aid in nitrogen removal from the patient
• if enzyme defect is after the synthesis of argininosuccinate, massive arginine supplementation has proved beneficial; helps with regenerating ornithine and continues the cycle → two NH3 excreted as argininosuccinate

pg 1566

Question 16

congenital hyperammonemia

Answer 16

• due to genetic defect in any of the enzymes of the urea cycle
• failure to incorporate ammonia into urea leads to high blood ammonia during the first weeks following birth
• high morbidity with neurologic manifestations and high mortality
• symptoms manifest shortly after birth
• if untreated, irreversible damage occurs

pg 1567

Question 17

acquired hyperammonemia

Answer 17

• due to ANY liver disease → viral hepatitis, hepatotoxin, alcoholism, cirrhosis, etc
• urea cycle is impaired leading to elevated levels of ammonia in the blood
• elevated blood ammonia is neurotoxic

pg 1567

Question 18

management of hyperammonemia

Answer 18

• low-protein diet (↑ ammonia, ↓ urea, ↑ glutamine)
• high dose glucose (IV dextrose)
• hemodialysis (filter ammonia from blood, remove need for urea)
• liver transplant (if SEVERE, completely new urea cycle)
• phenylbutyrate (forms phenylacetylglutamine which contains 2 atoms of nitrogen and is excreted in urine)

glutamine synthetase (“safety bucket”) → produced glutamine from whatever is not converted to urea

pg 1568