CLINICAL CASES
43
A nsw ers
[4.1]
E. Vitamin B12 is a cofactor in two biochemical reactions, the con-
version of homocysteine to methionine by the enzyme methionine
synthase and the conversion of L-methylmalonyl-CoA to succinyl-
CoA by methylmalonyl-CoA mutase. N5-methyl THF is a methyl
donor in the methionine synthase reaction. A folate deficiency would
result in decreased methionine synthase activity and decreases in
methionine and cystathionine concentrations, while homocysteine levels
would be increased. A vitamin B12 deficiency would also yield these
same results, but in addition methylmalonate levels would increase as
a consequence of a decrease in the activity of methylmalonyl-CoA
mutase activity.
[4.2]
B. Raw eggs contain a protein, avidin, which binds biotin strongly.
Because native avidin is resistant to hydrolysis by digestive pro-
teases, when it binds biotin it prevents its absorption. Avidin that has
been denatured by cooking will be broken down during the digestive
process. A biotin deficiency manifests itself as an erythematous, scaly
skin eruption and can also cause hair loss and conjunctivitis. A biotin
deficiency can also occur following prolonged total parenteral nutri-
tion if biotin is not supplemented.
[4.3]
A. The patient exhibits the classic symptoms of scurvy, a deficiency
in vitamin C. In addition to being an important biological antioxidant,
ascorbic acid is required for the hydroxylation of proline and lysine
residues of procollagen in the synthesis of collagen. A deficiency leads
to defects in collagen synthesis, which adversely affects the intercel-
lular cement substances in connective tissue, bones, and dentin.
[4.4]
D. The fetus needs a constant supply of cofactors for normal devel-
opment. Folic acid supplements of 400 |Jg/day prior to conception
have been shown to decrease the incidence of neural tube defects
such as spina bifida.
B IO C H E M IS T R Y PE A R L S
Folate (folic acid) is an essential vitamin that, in its active form of
tetrahydrofolate, transfers 1-carbon groups to intermediates in
metabolism and plays an important role in DNA synthesis.
THF is necessary for the de novo synthesis of purines and the conver-
sion of deoxyuridine 5'-monophosphate (dUMP) to deoxythymi-
dine 5'-monophosphate (dTMP).
The major metabolic perturbation in folate deficiency occurs in
megaloblastic anemia.
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