Ascorbic Acid

Question 1

Why can humans not synthesize ascorbic acid

Answer 1

lack gluconolactone oxidase (last enzymatic step)

Question 2

Vitamin C sources

Answer 2

citrus, papaya, cantaloupe, broccoli, bell peppers, strawberries

Question 3

How is Vit C absorbed

Answer 3

active transport

Question 4

What must occur before Vit C is absorbed

Answer 4

it has to be oxidized to dehyrdoascorbate

Question 5

What occurs to dehydroascorbate in enterocytes

Answer 5

reduction to ascorbic acid (dehydroascorbate reductase, requires reduced GSH)

Question 6

Where does absorption occur

Answer 6

primarily in distal portion of SI

Question 7

Is it better to have high intakes or low intakes of Vit C at a time

Answer 7

Low due to better absorption; 98% at low, 16% at high

Question 8

What are potential factors that can impair absorption

Answer 8

pectin, zinc, high iron content in GI (AA descruction)

Question 9

Absorption method for Vit C

Answer 9

sodium-independent carrier-mediated transport

Question 10

How does Vit.C transport in plasma

Answer 10

free form, moves freely into cells

Question 11

tissue and plasma level reflect intake until ~Xmg/day

Answer 11

90

Question 12

Explain the antioxidant mechanism

Answer 12

reducing agent
Reactive oxygen species: OH*, O2-, H2O2, HO2
Purpose: oxidize LDL and red blood cells

Question 13

Explain pro-oxidant mechanism

Answer 13

reacts with free Fe+3 or Cu+1

Question 14

Explain collagen transormation

Answer 14

Fe+2 required to hydroxylate proline to aggregate collagen molecule–> ascorbate reduces iron back to ferrous state

Question 15

Collagen transformation mechanism

Answer 15

Vit C required for hydroxylation of lysine residues –>permit cross-linking of collagen and other post-translational modifications

Question 16

T/F: Vit C required for 3 reactions in the synthesis of carnitine from trimethyllysine

Answer 16

False; only 2 reactions

Question 17

Vit C function in Tyrosine synthesis and metabolism mechanism

Answer 17

hydroxylation of phenylalanine to form tyrosine –>ability to make dopamine and norepinephrine

Question 18

Nutrient-Nutrient Interactions

Answer 18

Iron: increase absorption of nonheme iron
Copper: interfere with absorption and tranportation in doses above 600mg/d
Folate: Encourages reduced, active form

Question 19

Where is Vit C metabolized

Answer 19

liver

Question 20

How is vit C metabolized

Answer 20

oxidized to dehydroascorbate

Question 21

RDA

Answer 21

75mg women
90mg men\
+35mg for smokers

Question 22

UL

Answer 22

2 g/d

Question 23

Vitamin C intakes <10mg/d result in x

Answer 23

scurvy

Question 24

Symptoms of scurvy

Answer 24

petechiae, bleeding gums, easy brusing, joint pain, impaired healing, overgrowth of hair

Question 25

Is scurvy common

Answer 25

no; rare in the US

Question 26

Toxicity

Answer 26

likely with injection of several large doses than one single dose –>causes kidney stones

Question 27

How is Vit C assessed

Answer 27

blood serum level