Biomolecules: Homocystinuria and its associate treatments

Homocystinuria is an inherited disorder of the metabolism of the amino acid methionine. The normal metabolism of this substance requires the enzyme cystathionine beta synthase. The disease is inherited as an autosomal recessive trait, with approximately 1 in 100,000 infants being born with homocystinuria.

In individuals with homocystinuria, homocysteine, an amino acid, accumulates in the serum, leading to an increased excretion of homocysteine in the urine. High levels of homocysteine in the blood results in a multi-systemic disorder of the connective tissue, muscles, central nervous system, and the cardiovascular system. Fortunately, homocystinuria can be identified by a blood test at birth, which allows for preemptive treatment to be administered to prevent dramatic damage to the individual’s body. One way symptoms of the disease can be avoided is through a strictly reduced methionine diet. Another treatment comes in the form of high doses of pyridoxine, or vitamin B6.

Since the laboratory analysis of homocysteine is complicated by the fact that it exists bound to other thiol amino acids via disulfide bonds, it is first necessary to subject these disulfide bonds to reduction prior to analysis. It is necessary to reduce these disulfide bonds in order to get a true value of the quantity of homocysteine in a plasma sample. Why is this so?

(Choice A)
The true quantity of homocysteine would be artificially high if the reduction step were not carried out prior to analysis.
(Choice B)
It is necessary because disulfide bound homocysteine would otherwise precipitate out of the serum before analysis could be conducted.
(Choice C)
The true quantity of homocysteine would be artificially low if the reduction step were not carried out prior to analysis.
(Choice D)
It is necessary in order to prevent homocysteine from degrading in the sample.

MCAT

Course: MCAT > Unit 3

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