Water-Soluble Vitamins

FSHN 3600 - Exam 3

What are some of the key features that all water-soluble vitamins have in common?

• Often require membrane transporters

• Nontoxic

• Not stored

• Required frequently in diet

Which B vitamins are NOT involved in energy production?

Folate & B12

How are water-soluble vitamins generally absorbed?

Typically require a membrane transporter and rely on either active or facilitated diffusion

What B vitamins are involved in cell division?

B12, folate, choline

Equivalence is important because vitamins have different forms & not all are equal by ___. Therefore contribution to RDA can vary because some contribute more or less.

weight

Methyl trap in fortification cycle is the result of

B12 deficiency

How do water-soluble vitamins circulate in the body?

Circulate in bloodstream in either a free or protein-bound form.

Vitamin B6 active form

PLP

PMP

Biotin (B7) active form

Biotinidase

Folate active form

tetrahydrofolate polyglutamate (THF polyglut)

How are water-soluble vitamins excreted?

Readily excreted through urine when there is an excess.

Thiamin (B1) active form

thiamin pyrophosphate (TPP)

What is the reason behind vitamin B6 deficiency

Heme synthesis is affected

Thiamin (B1) function

Cofactor in oxidative decarboxylation & transketolase reaction (HMS).

Thiamin (B1) absorption

Facilitated diffusion; req: TPP-specific phosphatases to remove P groups then re-phosphorylate it in the cell

Thiamin (B1) transportation

Bound to albumin in circulation

Thiamin (B1) deficiency

Beriberi (dry, wet, infantile) or Wernicke-Korsakoff syndrome; dermatitis

Lack of TPP leads to buildup of substrates and alt. products in a pathway.

Increased pyruvate & lactate

Thiamin (B1) reactions/pathways

Glycolysis, TCA cycle, lipogenesis, glycogen metabolism, HMS (transketolase reaction)

Cofactor for PDH & alpha-KGDH

Thiamin (B1) RDA

mg/day

Riboflavin (B2) active form

FAD (H₂)

Riboflavin (B2) function

Cofactor in energy metabolism

Riboflavin (B2) deficiency

Ariboflavinosis: mouth and tongue lesions.

Riboflavin (B2) transportation

Active & passive diffusion

Riboflavin (B2) reactions/pathways

TCA cycle & beta-ox of FAs

Riboflavin (B2) RDA

mg/day

Niacin (B3) active form

Nicotinamide, NAD

Niacin (B3) function

FA synthesis & redox reactions

Niacin (B3) deficiency

Pellagra - 4Ds: dermatitis, dementia, diarrhea, & death

Niacin (B3) reactions/pathways

Glycolysis, TCA, & beta-ox of FAs

Pantothenate (B5) function

Activates a substrate, acetylation of histones.

Pantothenate active form

CoA

Pantothenate transport

active & passive

Biotin (B7) function

Cofactor of ACC, PC, & PCC; biotinylation of histones.

Pantothenate (B5) & Biotin (B7) deficiency

None, rare.

Vitamin B6 function

Forms a Schiff-base between enzyme & substrate

Vitamin B6 reactions/pathways

Transamination: gluconeogenesis

Decarboxylation: neurotransmitter synthesis & heme synthesis

Transulfuration: methionine —→ cysteine

Vitamin C function

Coupled with hydroxylation reactions

AA substrate

Decreases oxidative stress.

Vitamin C reactions/pathways

• Collagen synthesis

• Carnitine synthesis

• Neurotransmitter synthesis

Vitamin C assessment

Measured by white blood cell

Vitamin A gene expression is induced by ___.

glucagon

Folate function

DNA synthesis & provision of methyl groups as SAM.

Vitamin B12 function

Met synthesis & odd chain FA synthesis (methylmalonyl CoA —→ succinyl CoA)

Vitamin B6 deficiency

Microcystic anemia & neurological issues.

Folate deficiency

Megaloblastic anemia

Vitamin C deficiency

Scurvy

Vitamin B12 deficiency

Megaloblastic anemia/pernicious anemia

What types of tests are used to assess vitamin status?

Functional, nonfunctional, & load testing.

Functional test definition

Determines a measurable metric that directly reflects the function of a given vitamin.

Example of a functional test

An erythrocyte transketolase activity test measures intracellular transketolase activity to determine the status.

Non-functional test definition

Test assesses the abundance of a vitamin in a blood or urinary sample.

Example of a nonfunctional test

Status of niacin can be determined from a urinary N-methylnicotinamide. 

Load test definition

Focuses on the capacity of a given pathway to metabolize a large load of substrate. After a dose of substrate is given, the amount of a specific metabolite is measured.

Example of a load test

Status of Vitamin B6 can be determined by administering oral doses of tryptophan and measuring the xanthurenic acid in urine.

Thiamin (B1) Assessment

EKTA

• relies on function of TPP (cofactor) for enzymes in HMS

• 2 samples; Add TPP to 1 sample of lysed RBCs

• Increase of >25% in activity = deficiency

Riboflavin (B2) assessment

Glutathione reductase test

• 2 samples; Add FAD to 1 sample of RBCS

• Measures GSH reductase enzyme activity

• Determine coefficient: GSH reductase + FAD / GSH reductase

• >1.4 = deficiency

Niacin (B3) Assessment

Urinary N-methylnicotinamide test

Pantothenate Assessment

Urinary and blood (plasma) concentrations of pantothenic acid.

Vitamin B6 Assessment

Load test

• Oral doses of methionine or tryptophan

• Deficiency = increased homocysteine or xanthurenic acid in blood/urine

Folate Assessment

Serum folate — diet (what you ate in last few days)

RBC folate — reflects intracellular folate

Vitamin B12 Assessment

Methylmalonic acid in urine.