Water-Soluble Vitamins Flashcards
The only glucose-derived vitamin
Ascorbic acid (Vitamin C)
What is the role of ascorbic acid in collagen synthesis?
Ascorbate (anion of ascorbic acid) is the cofactor for prolyl hydroxylase (which hydroxylates proline in procollagen) and lysyl hydroxylase (which hydroxylates lysine in procollagen).
How does vitamin C deficiency cause poor wound healing?
Low vitamin C causes insufficient/defective collagen synthesis so that the break in the skin cannot be resealed fully.
How does vitamin C deficiency cause osteoporosis?
Organic bone matrix consists largely of collagen so low vitamin C can lead to defective bone formation.
How does vitamin C deficiency cause capillary fragility?
The ground substance of capillary walls contain collagen. With low vitamin C, less collagen is produced, weakening the capillary walls. This results to easy bruising, petechial hemorrhages in skin and mucous membranes.
How does vitamin C deficiency cause anemia?
Because vitamin C helps increase intestinal Fe absorption (by reducing Fe3+ to Fe2+), vitamin C deficiency can lead to Fe deficiency and anemia.
How does ascorbic acid increase intestinal Fe absorption?
It converts Fe3+ (ferric) to Fe2+ (ferrous), which is more readily absorbed in the intestines.
With regard to vitamin C consumption, why is it advisable to eat fruits and vegetables fresh?
Vitamin C in food is destroyed by overcooking and prolonged storage.
What is the role of vitamin C in catecholamine synthesis?
It is the cofactor in the conversion of dopamine to epinephrine:
- Tyrosine –> DOPA (3,4,-dihydroxyphenylalanine) via tyrosine hydroxyalse
- DOPA –> Dopamine via DOPA decarboxylase
- Dopamine –> Epinephrine via Dopamine beta-hydroxylase (WITH ASCORBIC ACID AS COFACTOR)
- Epinephrine –> Norepinephrine via phenylethanolamine N-methyl transferase with SAM (S-adenosylmethionine) as methyl donor.
True/False: It is common to have deficiency of a single vitamin.
False. Deficiency of a single vitamin is rare. Poor diets often associate with multiple deficiency states and overlapping symptoms.
True/False: Toxicities from vitamins are rare.
True. Vitamins are stored in tissue at low levels. Excess vitamins (those that exceed the renal threshold) are excreted.
What is the role of vitamin C in carnitine synthesis?
It acts as cofactor in the 4 reactions that convert epsilon-N-trimethyllysine to L-carnitine (preceeded by trimethylation of lysine).
- Lysine residues in proteins are trimethylated at the epsilon-amino group to make epsilon-N-trimethyllysine.
- WITH VITAMIN C AS COFACTOR, 4 steps follow that convert epsilon-N-trimethyllysine to L-carnitine.
How does vitamin C deficiency cause fatigue?
Vitamin C helps produce carnitine, which transports fatty acids from the cytosol to the mitochondria for beta oxidation. Fatty acid oxidation produces ATP. So without enough vitamin C, not enough ATP (energy) can be produced, hence the fatigue.
What is the role of vitamin C in tyrosine degradation?
It is a cofactor of p-hydroxyphenylpyruvate dioxygenase, which converts p-hydroxyphenylpyruvate to homogentisate.
- Tyrosine –> p-hydroxyphenylpyruvate via tyrosine aminotransferase (deaminated)
- p-hydroxyphenylpyruvate –> homogentisate via p-hydroxyphenylpyruvate dioxygenase WITH VITAMIN C AS COFACTOR
- Series of reactions that convert tyrosine into either acetoacetate (as a ketogenic amino acid) or fumarate (as a glucogenic amino acid).
What is neonatal tyrosinemia?
A disorder due to p-hydroxyphenylpyruvate dioxygenase. A benign condition in newborns; treated with dietary protein restriction and ascorbate supplementation (since enzyme requires ascorbate for its activity).
What happens when there is excess of vitamin C?
High excretion of kidney stones from calcium salt deposits of oxalate, a breakdown product.
It is the biologically active form of vitamin B1.
Thiamine pyrophosphate (TPP) or Thiamine diphosphate (TDP).
How is TPP formed?
PPP synthase transfers a phosphate group from ATP to thiamine.
What is the role of TPP in PPP (pentose phosphate pathway)?
It is a cofactor in the transketolation reactions (non-oxidative stage of PPP):
- Transketolation between xylulose 5-PO4 and ribose 5-PO4 to form seduheptulose 7-PO4 and glyceraldehyde 3-PO4
- Transketolation between xylulose 5-PO4 and erythrose 4-PO4 to form glyceraldehyde 3-PO4 and fructose 6-PO4.
What is the role of TPP in TCA?
TPP is a cofactor of the various dehydrogenases in the oxidative decarboxylations of the TCA:
- pyruvate –> acetyl CoA via pyruvate dehydrogenase
- isocitrate –> alpha-ketoglutarate via isocitrate dehydrogenase
- alpha-ketoglutarate –> succinyl CoA via alpha-ketoglutarate dehydrogenase
How does TPP deficiency impair cellular function?
Without the key energy-synthesizing pathways in TCA that uses TPP as cofactor, ATP won’t be formed. Low ATP levels impair cellular function.
How does TPP deficiency cause lactic acidosis?
TPP is a cofactor of pyruvate dehydrogenase, which converts pyruvate to acetyl CoA. TPP deficiency impairs this conversion so that there is accumulation of pyruvate. Since pyruvate is convertible to lactate (via lactatde dehydrogenase), lactate also accumulates. This could lead to lactic acidosis.
How does TPP deficiency cause peripheral neuropathy?
TPP is a cofactor in key energy-synthesizing pathways (TCA) that are also important in the CNS (since ATP used up in neuronal depolarization must be replenished). Actually, glutamate (a neurotransmitter), may be oxidized in nerve cells to yield alpha-ketoglutarate (a substrate in TCA). TPP deficiency therefore impairs nerve transmission in the neural tissue by lowering ATP synthesis.
How does TPP deficiency cause lactic acidosis?
TPP is a cofactor of pyruvate dehydrogenase, which converts pyruvate to acetyl CoA. TPP deficiency impairs this conversion so that there is accumulation of pyruvate. Since pyruvate is convertible to lactate (via lactatde dehydrogenase), lactate also accumulates. This could lead to lactic acidosis.
What is the role of TPP in the catabolism of branch-chained amino acids?
TPP is a cofactor of alpha-ketoacid dehydrogenase, which catabolizes branch-chain amino acids (valine, isoleucine, and leucine) by oxidative decarboxylation:
- valine, isoleucine –> succinyl CoA –> glucogenic precursor
- leucine –> acetyl CoA and acetoacetate –> ketogenic precursor
How is thiamine deficiency diagnosed?
RBC transketolase (enzyme) activity is low when there is TPP deficiency. Thus, accelerated RBC transketolase activity upon addition of TPP indicates a deficiency.
What is Wernicke-Korsakoff syndrome?
A thiamine deficiency associated with chronic alcoholism and drug abuse.
What is Beriberi?
A severe thiamine deficiency syndrome found among Asians whose staple food is rice. If the rice is well-polished (i.e. rice bran was removed), its thiamine content is depleted.
