Posts: 3,675,937
Threads: 734,344
Joined:
Sep 2021
Reputation:
5
An adolescent female develops hemiballismus (repetitive throwing motions of the arms) after anesthesia for a routine operation. She is tall and lanky, and it is noted that she and her sister both had previous operations for dislocated lenses of the eyes. The symptoms are suspicious for the disease homocystinuria (236300). Which of the statements below is descriptive of this disease?
Options:
1. Patients may be treated with dietary supplements of vitamin B12
2. Patients may be treated with dietary supplements of vitamin C
3. There is deficient excretion of homocysteine
4. There is increased excretion of cysteine
5. There is a defect in the ability to form homocysteine from methionine by methylation
Posts: 3,675,937
Threads: 734,344
Joined:
Sep 2021
Reputation:
5
111. vit B12 can convert homocysteine to methionine... (guess)
Posts: 3,675,937
Threads: 734,344
Joined:
Sep 2021
Reputation:
5
The Correct Answer is: There is a defect in the ability to form homocysteine from methionine by methylation
Explanation:
In the synthesis of cysteine, the following sequence of steps occurs, where SAM is S-adenosylmethionine, CS is cystathionine synthase, cys is cysteine, and α-KG is α-ketoglutarate: methionine → SAM → homocysteine + adenosine homocysteine + serine → cystathionine → cys + α-KG + NH2+ (CS, B6). Cystathionine synthetase, a pyridoxal phosphate (vitamin B6) enzyme, catalyzes the condensation of serine and homocysteine to form cystathionine. A deficiency of this enzyme leads to a buildup of homocysteine, which oxidizes to form homocystine. This may result in mental retardation, but sometimes causes dislocated lenses and a tall, asthenic build reminiscent of Marfan™s syndrome. Patients with homocystinuria also have a clotting diathesis, requiring care to avoid dehydration during anesthesia. Their cysteine deficiency must be made up from dietary sources. In some cases, dietary intake of vitamin B6 (pyrixodal phosphate) may alleviate symptoms because of its requirement by the crucial enzymes
Posts: 3,675,937
Threads: 734,344
Joined:
Sep 2021
Reputation:
5
i have a doubt here... the dis. is homocystinuria..... that is there is excess of homocysteine.. so it should occur with
1) defect in conversion of homocysteine to methionine (B12 dependent) or
2) defect in conversion of homocysteine to cystathionine (B6 dependent)
how could it be defect to form homocysteine cause methionine??
Posts: 3,675,937
Threads: 734,344
Joined:
Sep 2021
Reputation:
5
i have a doubt here... the dis. is homocystinuria..... that is there is excess of homocysteine.. so it should occur with
1) defect in conversion of homocysteine to methionine (B12 dependent) or
2) defect in conversion of homocysteine to cystathionine (B6 dependent)
how could the defect in the ability to form homocysteine cause homocystinuria???
Posts: 3,675,937
Threads: 734,344
Joined:
Sep 2021
Reputation:
5
am i the only person with this doubt.. plz someone explain!!
Posts: 3,675,937
Threads: 734,344
Joined:
Sep 2021
Reputation:
5
homocystenemia caused by two enzyme deficiencies :
a]cystathionine beta synthase deficiency
b]methyl tetra hydrofolate -homocysteine methyl transferase deficie. Elevations of homocysteine also occur in the rare hereditary disease homocystinuria and in the methylene-tetrahydrofolate-reductase polymorphism genetic traits. The latter is quite common (about 10% of the world population) and it is linked to an increased incidence of thrombosis and cardiovascular disease and that occurs more often in people with above minimal levels of homocysteine (about 6 μmol/L).
the question above tells us the defect in the ability to form homocysteine from methionine by methylation which is correct.i guess carefully go through the pathway as to how homocysteine is formed from s-adenosyl methionine .
Posts: 3,675,937
Threads: 734,344
Joined:
Sep 2021
Reputation:
5
defect in methyl tetra hydrofolate homocysteine methyl transferase deficiency... causes defect in methylation of homocysteine to form methionine... so the 5th option should have been
5. There is a defect in the ability to form methionine from homocysteine by methylation..
instead of
5. There is a defect in the ability to form homocysteine from methionine by methylation
anyway treatment for this is diet low in methionine so that it could not be converted to homocysteine through methionine - sam - homocysteine pathway..
thanx for ur responses kola..
