Study Notes on Vitamins and Trace Elements

Different chemical compounds that show the same biological activity are collectively known as vitamers.

Fat-soluble vitamins (e.g., A, D, E, and K) are absorbed dissolved in lipid micelles.
Water-soluble vitamins are present in foods bound to proteins, and their release may require the action of gastric acid.

Ligand binding assays (e.g., radioimmunoassays)
High-performance liquid chromatography (HPLC)
Microbiological assay
Measurement of plasma, urine, or tissue concentrations of vitamins and their metabolites.

Vitamin A is a generic term referring to compounds with the biological activity of retinol.
Functions of Vitamin A:
- Required for normal vision.
- Essential for reproduction and embryonic development.
- Important for cell and tissue differentiation and immune function.
Types of Vitamin A:
- Retinoids:
  - Retinol
  - Retinaldehyde
  - Retinoic acid
- Carotenoids:
  - Alpha-carotene
  - Beta-carotene
  - Beta-cryptoxanthin
  - Lycopene, lutein, zeaxanthin

Vitamin D is metabolized to a biologically active form that functions as a steroid hormone.
Functions of Vitamin D:
- Maintains calcium homeostasis.
- Maintains phosphorus homeostasis.
Forms of Vitamin D:
- Vitamin D2 (ergocalciferol): from the plant steroid ergosterol.
- Vitamin D3 (cholecalciferol): from radiant energy from the sun striking the skin, converting the precursor 7-dehydrocholesterol.

Vitamin D3 (Cholecalciferol):
- 25-Hydroxycholecalciferol (Calcidiol): Mr = 384.6
- 1α-Hydroxycholecalciferol
- 24,25-Dihydroxycholecalciferol
- 1,25-Dihydroxycholecalciferol (Calcitriol): Mr = 416.6
- 1,24,25-Trihydroxycholecalciferol
Vitamin D2 (Ergocalciferol):
- 25-Hydroxyergocalciferol (Ercalciol): Mr = 412.6
- 1(S)-Hydroxycalciol (1α(OH)D3): Mr = 400.6
- 24(R)-Hydroxycalcidiol (24,25(OH)2D3): Mr = 416.6
- 1,25-Dihydroxyergocalciferol: Mr = 428.6
- 1,24,25-Trihydroxyergocalciferol (Ercalcitetrol): Mr = 432.6

Functions of Vitamin E:
- Acts as a radical trapping antioxidant in membranes.
- Cell signaling by inhibition or inactivation of protein kinase C, and modulation of gene expression.
- Inhibits cell proliferation and platelet aggregation.
Types of Tocopherols and Tocotrienols:
- Tocopherols:
  - Saturated side chain; occur naturally as free alcohols.
  - Variants: α, β, γ, δ differ in methylation of the chromanol ring.
- Tocotrienols:
  - Unsaturated side chain; occur in foods as both free alcohols and esters.
  - Variants: α, β, γ, δ differ in methylation.

Required for the synthesis of several proteins necessary for blood clotting.
Food sources include cabbage, cauliflower, spinach, and other green leafy vegetables, as well as cereals.

Functions as a coenzyme in energy-yielding metabolism.
Acts as a prosthetic group of cryptochromes in the eye.
Found in foods like cereal, nuts, milk, eggs, green leafy vegetables, and lean meat.

Precursor of the nicotinamide moiety of the nicotinamide nucleotide coenzymes: NAD and NADP.

Involves naturally occurring pyridine derivatives: pyridoxine (pyridoxol), pyridoxal, and pyridoxamine.
Plays a role in metabolism of proteins, lipids, and carbohydrates.

Folic acid and folate are often used interchangeably.
Folic acid refers to the oxidized compound pteroyl monoglutamate, while various tetrahydrofolate derivatives are known as folates.

Cyanocobalamin is the most stable pharmacological form.
Other forms:
- Aquacobalamin or hydroxocobalamin: formed by photolysis of cyanocobalamin.
- Methylcobalamin: the predominant form in human plasma and cytosol.
- Adenosylcobalamin: the major vitamer in tissues such as the liver.

The physiologically important compound is L-ascorbic acid.
It can undergo oxidation to form monodehydroascorbate free radical and dehydroascorbate, both of which have vitamin activity.
Not a naturally occurring compound is erythorbic acid.
Functions:
- Cofactor for various redox reactions.
- Necessary for collagen synthesis, neurotransmitters, protein metabolism, as an antioxidant in the immune system, and for wound healing.

Exists in complexes with proteins as heme proteins (hemoglobin or myoglobin), heme enzymes, or non-heme compounds (transferrin and ferritin).
Important for cellular processes like oxygen transport, electron transfer, oxidation-reduction reactions, mitochondrial respiration, and DNA synthesis.
Exists in two forms: reduced ferrous (Fe2+) and oxidized ferric (Fe3+).

Plays catalytic and structural roles in proteins, and other biomolecules.
Acts as a cofactor for several antioxidant enzymes such as ceruloplasmin, monoamine oxidase, cytochrome oxidase, ascorbic acid oxidase, and superoxide dismutase.

The human selenoproteome contains 25 selenoproteins, including:
- Glutathione peroxidases
- Thioredoxin reductases
- Iodothyronine deiodinases
Functions include:
- Defense against oxidative stress
- Maintenance of cellular redox status
- Role in immune and anti-inflammatory reactions, cell proliferation, apoptosis, thyroid hormone metabolism, fertility mechanisms, and detoxification of harmful substances.

Chemical affinity for cellular thiol, imidazole, and carboxyl ligands that maintain the tertiary structural orientation of proteins.
Many metalloenzymes rely on zinc for optimal catalytic roles and enforced stereochemistry or electronic state around the metal.

Trivalent chromium is essential for normal carbohydrate, lipid, and protein metabolism.
Behaves biologically active as part of an oligopeptide – chromodulin that potentiates the effect of insulin by facilitating insulin binding to receptors at the cell surface.

Exists in the body as an ion and is biologically inactive unless complexed by a special cofactor.
Each human enzyme contains a pterin-based Mo cofactor (Moco) at its active site, involved in the decomposition of metabolites for renal excretion.