Page 113 - First Aid for the USMLE Step 1 2020, Thirtieth edition [MedicalBooksVN.com]_Neat

BIOCHEmISTRY ``BIOCHEMISTRY—NUTRITION BIOCHEmISTRY ``BIOCHEMISTRY—NUTRITION SECTION II 69

Also called cobalamin.
Vitamin B 12
FUNCTION Cofactor for methionine synthase (transfers Found in animal products.
CH 3 groups as methylcobalamin) and Synthesized only by microorganisms. Very large
methylmalonyl-CoA mutase. Important for reserve pool (several years) stored primarily in
DNA synthesis. the liver. Deficiency caused by malabsorption
DEFICIENCY Macrocytic, megaloblastic anemia; (eg, sprue, enteritis, Diphyllobothrium latum,
hypersegmented PMNs; paresthesias achlorhydria, bacterial overgrowth, alcohol
and subacute combined degeneration excess), lack of intrinsic factor (eg, pernicious
(degeneration of dorsal columns, lateral anemia, gastric bypass surgery), absence of
corticospinal tracts, and spinocerebellar tracts) terminal ileum (surgical resection, eg, for
due to abnormal myelin. Associated with Crohn disease), certain drugs (eg, metformin),
 serum homocysteine and methylmalonic or insufficient intake (eg, veganism).
acid levels, along with 2° folate deficiency. Anti-intrinsic factor antibodies diagnostic for
Prolonged deficiency Ž irreversible nerve pernicious anemia.
damage. Folate supplementation can mask the
hematologic symptoms of B 12 deficiency, but
not the neurologic symptoms.

Protein Fatty acids with odd number of
carbons, branched-chain amino acids
THF Methionine SAM
CH to anabolic
3
pathways Methylmalonyl-CoA
B Methionine synthase S-adenosyl B 12 Methylmalonyl-CoA
mutase
12 homocysteine
Succinyl-CoA
B 6
THF–CH Homocysteine
3
Heme TCA
B Adenosine
6
Cysteine

Vitamin C Also called ascorbic acid.

FUNCTION Antioxidant; also facilitates iron absorption Found in fruits and vegetables.
2+
by reducing it to Fe state. Necessary Pronounce “absorbic” acid.
for hydroxylation of proline and lysine in Ancillary treatment for methemoglobinemia by
3+
2+
collagen synthesis. Necessary for dopamine reducing Fe to Fe .
β-hydroxylase, which converts dopamine to
NE.
DEFICIENCY Scurvy—swollen gums, easy bruising, Vitamin C deficiency causes sCurvy due to a
petechiae, hemarthrosis, anemia, poor wound Collagen synthesis defect.
healing, perifollicular and subperiosteal
hemorrhages, “corkscrew” hair.
Weakened immune response.

EXCESS Nausea, vomiting, diarrhea, fatigue, calcium
oxalate nephrolithiasis. Can  iron toxicity in
predisposed individuals by increasing dietary
iron absorption (ie, can worsen hereditary
hemochromatosis or transfusion-related iron
overload).

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