Water Soluble Vitamins

Question 1

What are the components of the vitamin C group?

Answer 1

Ascorbic Acid & L-Ascorbic Acid

Question 2

What is the oxidised form of vitamin C called?

Answer 2

Dehydroascorbic Acid

Question 3

Why is vitamin C an essential vitamin in the diet?

Answer 3

Because humans do not possess the enzyme that is required to synthesise ascorbate

L-gulonolactone-oxidase

Question 4

What is important to remember about vitamin C’s absorption?

Answer 4

In diets up to 100mg/day absorption is high at 80-95%

Once you consume more than 100mg/day the absorption rate decreases rapidly due to transporter saturation

Question 5

How is excess vitamin C excreted out of the body?

Answer 5

Excess vitamin C is excreted via the urine mostly (but some stays in the lumen, metabolised by bacteria, excreted in faeces

Question 6

How is vitamin C absorbed?

Answer 6

Mostly via SVCT
Sodium dependent vitamin C transporter

Dehydroascorbic acid can be absorbed via glucose transporters (GLUTs)

Question 7

Why is it important to highlight vitamin C absorption via GLUTs?

Answer 7

These are sodium independent

They are competitively inhibited by hexose sugars

There is a link to diabetes with low absorption -> i.e. insulin insensitivity

Question 8

Where is vitamin C found in the body?

Answer 8

70% in plasma & RBCs

30% in WBCs & other tissues

Question 9

What is the difference between the ascorbic acid found in the plasma, RBCs and WBCs?

Answer 9

Plasma - bound to albumin

RBCs - unconcentrated

WBC - concentrated

Question 10

Name some body tissues where you expect high volumes of ascorbic acid

Answer 10

Liver & Skeletal Muscle due to their size

Also found in the adrenals & pituitary

Question 11

What is the half life of vitamin C?

Answer 11

10-20 days (remember as dietary intake increases, half life decreases)

Question 12

Name 2 urinary metabolites of vitamin C

Answer 12

45% = oxalic acid 
20% = 2,3-diketogulonic acid
1% = L-ascorbate sulphate

Question 13

Why can urine be used to measure vitamin C intake?

Answer 13

In diets that are vitamin C complete kidneys excrete absorbic acid unmetabolised.

At plasma levels of 70-85micromoles/l the renal system (lecture says excretion, I think reabsorption) is saturated and subsequent excretion is proportional to dietary intake

Question 14

What are the 3 main metabolic functions of vitamin C?

Answer 14

Enzyme Cofactors
Enhances non-haem iron absorption
Acts as an antioxidant

Question 15

Give 3 specific metabolic functions of vitamin C acting as a cofactor

Answer 15

Increases collagen, carnatine and catecholamine synthesis

Decreases DNA & Histone methylation - important for epigenetics

Decreases hypoxia-inducible factor - important for gene transcription

Question 16

What is the common mechanism of vitamin C as an enzyme cofactor?

Answer 16

Vitamin C acts as a cofactor for lots of enzymes, particularly hydroxylase enzymes.

There are two key groups of hydroxylase enzymes important here:
Alpha-ketoglutarate-Linked Iron Containing hydroxylases
Copper Containing hydroxylases

What absorbic acid does is donate electrons to the metallic core of the enzyme to keep them in their reduced state which makes the enzyme active and speeds up hydroxylation reactions
(E.g. Fe2+ -> Fe3+)

Question 17

Give two metabolic synthetic pathways/products that require vitamin C’s hydroxylase properties

Answer 17

Collagen & Carnitine Synthesis

Question 18

How is collage synthesis related to hydroxylation & vitamin C?

Answer 18

Collagen is composed of three interwoven chains made up of glycine, proline and hydroxyproline

Hydroxyproline is essential for the stability of collagen & requires vitamin C to activate proline hydroxylase

To give the collagen strength hydroxylysine can form cross links between chains and requires vitamin C to activate lysine hydroxylase

Question 19

How is vitamin C involved in carnitine synthesis?

Answer 19

Activation of 2 enzymes essential for 2 steps
Trimethyllysine Hydroxylase
y-butyrobetaine hydroxylase

Question 20

How does vitamin C increase non-haem iron absorption?

Answer 20

When vit C is present in the stomach with non-haem iron it is able to reduce it. Reduced iron is less able to form insoluble complexes with phytates and therefore more available to absorption

Question 21

What 3 ways does vitamin C act as an antioxidant?

Answer 21

Can donate electrons directly to ROS & RNS

Able to regenerate vitamin E and glutathione

Question 22

What is the common known condition of vit C deficiency?

Answer 22

Scurvy

Question 23

Give 4 features of vitamin C deficiency other than scurvy

Answer 23

Cutaneous Haemorrhage
Improper bone development 
Weak Cartilage 
Poor Collagen Production 
Muscle Degradation 
Anaemia (Fe deficient)
Stunted Growth 
Susceptible to Infection

Question 24

How long does it take to show signs of Vit C deficiency with inadequate intake?

Answer 24

4-6months

Question 25

How much vitamin C do you need to prevent scurvy & allow for adequate wound healing

Answer 25

10mg/day = scurvy
20mg/day = wound healing

Question 26

What is the UK & USA guidelines on vitamin C?

Answer 26

UK = 40mg/day (+10 preg; +30 lac) based on preventing scurvy

USA = 90mg men, 75mg women (+35mg smoker) based on maximising neutrophil conc. and minimising urine excretion

Question 27

Name 4 rich sources of vitamin C

Answer 27

Rosehips
Parsley 
Black Currants
Peppers
Papaya 
Sprouts

Question 28

Give 4 things that make vitamin C liable

Answer 28

Easily destroyed by the following: oxygen, metal ions, alkaline solutions, heat and light

Question 29

Name one of the two enzymes involved in vitamin C degradation within food sources

Answer 29

Ascorbic Acid Oxidase

Phenolase

Question 30

At what amount does vitamin C become toxic?

Answer 30

> 1g/day over chronic course

Question 31

Give 3 features of vitamin C toxicity

Answer 31

Kidney Stones
Iron Overloading
GI upset (cramps and diarrhoea from the acidic loading)
Colon cancer misdiagnosis (FOB)
May have pro-oxidant activity (i.e. too much may result in too much ascorbate radicals)