390
CASE FILES: BIOCHEMISTRY
A nsw ers
[42.1]
C. para-Hydroxyphenylpyruvate is the a-keto acid cognate of tyro-
sine and is not affected by cobalamin levels. All others result from
cobalamin deficiency.
[42.2]
D. Cobalamin is transported in the blood by transcobalamins.
Cyanocobalamin, the pharmacologic preparation of cobalamin avail-
able in vitamin pills, is an active as is. Cobalamin is not active with an
iron cofactor. The intrinsic factor is produced by parietal cells in the
stomach. Cobalamin must be reduced to the Co+ state for activity.
[42.3]
B. Excess folate, by overwhelming the folate pool trapped as N5-
methyltetrahydrofolate, can allow for formation of N5,N10-methyl-
enetetrahydrofolate which is required for the thymidylate synthase
reaction for DNA synthesis and red blood cell formation. Folate is not
recognized as a methyl donor by methionine synthase. Folate does
not inhibit destruction of erythrocytes. Cobalamin is an important
critical vitamin not synthesized by humans.
B IO C H E M IS T R Y PE A R L S
Vitamin B12 (cobalamin) plays a critical role in DNA synthesis and
neurologic function.
Cobalmin deficiency can lead to a wide spectrum of hematologic,
neuropsychiatric, and cardiovascular disorders that can often be
reversed by early diagnosis and prompt treatment.
Cobalamin absorption from the gastrointestinal tract requires the
presence of a protein (the intrinsic factor, IF) secreted from the
parietal cells of the stomach to bind cobalamin and aid in its
absorption in the ileum.
REFERENCES
Devlin TM, ed. Text book of Biochemistry with Clinical Correlations, 5th ed,
New York: Wiley-Liss, 2002:795-7, 1154-7.
Issalbacher K, et al. Harrison’s Principles of Internal Medicine, 13th ed, New York:
McGraw-Hill, 1994:1726-32.
Scriver CR, Beaudet AL, Sly WS, et al. The Metabolic and Molecular Basis of
Inherited Disease, 8th ed, New York: McGraw Hill, 2001:2165-93.
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