Niacin (B3), Biotin, & Vitamin C

Who and from where was nicotinic acid (one form of Niacin) isolated from?

Yeast

Who cured pellagra?

Spies cured human pellagra using nicotinamide.

What are the food forms of Niacin?

Nicotinic Acid (in plants), Nicotinamide (in animals because it’s the storage form), and Nicotinamide Riboside (cow’s milk).

What are good dietary sources of Niacin?

Meat, wheat, cereals (because of fortification).

Niacin Absorption

• Mechanism of absorption for low concentrations (dietary) is not known.

• Appears to be a high-affinity acidic pH-dependent, carrier-mediated mechanism.

• Mechanism for pharmacologic doses is passive diffusion.

Niacin Transport

• Main circulating forms are free Nicotinic Acid and Nicotinamide.

• NOT bound to proteins.

• Sometimes bound to Immunoglobulins A,G,&M.

Niacin Tissue Uptake

• Mostly by Passive Diffusion!

• Erythrocytes, Kidney Cells, and Brain Cells use transporters.

Niacin Excretion

Excreted as 1-methylnicotinamide.

Niacin Functions

• Is the essential component of NADH and NADPH soluble electron carriers.

• NADH and NADPH serve as coenzymes for over 200 enzymes, including…

  • Glycolysis (G-3-PD)

  • Oxidative carboxylation of pyruvate

  • Oxidation of acetate in Citrate Cycle

  • Oxidation of ethanol

  • Beta-oxidation of Fatty Acids

Participates in REDOX Reactions like Riboflavin

NAD+

• A di-nucleotide

• Accepts a Hydride ion (equivalent of a proton and 2 electrons at the same time).

• Electron Transport Chain → FADH2 is covalently bound, hands e-’s off to NAD+.

Non-redox Function of NAD

• NAD serves as the substrate for ADP Ribosylation, a type of post-translation modification (adding to protein after it is made in translation).

• This adds an ADP Ribosyl group to enzymes to modulate/mediate/control their activity.

• ADP Ribosylation is catalyzed by Mono ADP Ribosyl Transferase and Poly Ribosyl Polymerase.

Poly ADP-Ribose Polymerases

• Family of proteins involve in DNA repair, differentiation, and apoptosis.

• Functions:

  • Modifies nuclear proteins by poly ADP ribosylation

  • DNA Repair

  • Activated in Response to UVB Radiation

• Impaired PARP function likely contributes to skin’s sensitivity to sun in pellagra.

Biosynthesis of Niacin

Nearly all animal species can synthesize niacin, NADH, and NADPH from amino acid Tryptophan.

Problem: Most people do not get enough tryptophan in diet.

Niacin Equivalent

Calculation used to account for contribution of dietary tryptophan to niacin.

60mg of tryptophan makes 1 mg of niacin (which equals 1 niacin equivalent).

Assessment of Niacin Deficiency

Urinary excretion of less than 0.8mg/d suggests deficiency.

Niacin Deficiency

• Pellagra

• Endemic in US in early 1900s:

  • Mainly in rural South in black individuals and women.

• Seen most in malnourished individuals, alcoholics, the poor, and the elderly.

• Still observed in developing countries with a corn dependency.

  • Corn is a poor source of niacin and tryptophan.

Clinical Symptoms of Niacin Deficiency

• Dermatologic Changes

  • On face, neck, back of hands, and forearms.

  • Skin cracking, desquamation, and hyperkeratosis.

• GI Lesions

  • Angular stomatitis, glossitis, achlorhydria.

• Neurologic Symptoms

  • Anxiety, depression, fatigue.

• Other

  • Anemia!

What is the RDA for Niacin?

RDA = 15mg

UL = 35mg

Increased needs for Pregnancy and Lactation.

Niacin Toxicity

• Short term: vasodilation, burning sensation, vomiting, etc.

• Long term: hyperpigmentation, abnormal glucose tolerance, peptic ulcers, hepatomegaly.

• Increased homocysteine.

• Larger doses: more than 10g/day leads to liver damage.

Niacin and Cholesterol

• Nicotinic Acid can lower LDL cholesterol so many people take it instead of Statins.

• Most are taking extremely high doses which can lead to toxicity in the long-term.

Biotin History

• 1901- Wilder observed that yeast requires a special substance to grow that he named Bios.

• 1927- Raw egg whites produced dermatitis but could be cured by a protective factor found in liver.

• 1935- Crystalline growth factor from dried egg yolk, named Biotin.

• 1940- Gyorgy concludes that Vitamin H and biotin are identical and isolated from liver.

Biotin Structure

Ureido Ring + Thiophene Ring (with sulfur group) + Valeric Acid Side chain (fatty acid)

What are dietary sources of Biotin?

Calf Kidney, Eggs, Molasses, Rapeseed Meal, Yeast.

Biotin Absorption

• Food forms of biotin have a Lysine bound to the Valeric Acid side chain.

  • Lysine must be removed before absorption.

• Biotinidase (enzyme) removes Lysine.

• SMVT (Sodium-Dependent Multvitamin Transporter) takes into cells.

Biotin Transport

Main circulating form (80%) is free biotin.

~20% bound to albumin, Alpha-globulin, and Beta-globulin.

Biotin Tissue Uptake

• Taken up by liver and other tissues via SMVT.

• Leukocytes take up biotin via Monocarboxylate Transporter-1 (MCT-1) and Solute Carrier Family 19 Member 3 (SLC19A3).

Biotin Excretion

Excreted in urine as biotin and biotin metabolites.

Biotin Functions

• Coenzyme for Carboylases, transferring single-carbon units as CO2.

  • Pyruvate carboxylase

  • Propinyl-CoA carboxylase

  • Acetyl CoA carboxylase I & II

  • Beta-Methylcrotonyl-CoA carboxylase

• Carrier for CO2- CO2 binds to the Ureido Ring.

Role of Biotin in Pyruvate Carboxylase

• 2 active sites

• Biotinyl-Lysine tether (ball and chain) moves CO2 from site 1 to site 2.

• CO2 is cleaved leaving Oxaloacetate

Role of Biotin in Propionyl CoA Carboxylase

• Catabolism of Isoleucine, Threonine, and Methionine

• Add CO2 to Propionyl-CoA to form Methylmalonyl-CoA

Role of Biotin in Acetyl CoA Carboylase I&II

Involved in first committed step of Fatty Acid Synthesis

Biotin transfers CO2

Role of Biotin in Beta-Methylcrotonyl-CoA Carboxylase

Leucine Catabolism

Assessment of Biotin Status

Urinary biotin and biotin metabolites

Biotin Deficiency

• VERY RARE

• Occur in individuals who ingest raw eggs in excess.

• Genetic defects in biotinidase (enzyme that removed biotinidase).

• Individuals with GI disorder.

Clinical Symptoms of Biotin Deficiency

• Hair thinning, loss of hair color, rash-scaly and red skin, depression, lethargy, hallucinations.

• In infants: hypotonia, developmental delay, intellectual disability.

  • Biotinidase defect- can live normal life on high dose biotin without lysine.

What is the DRI for biotin?

60 micrograms! No toxicity!

Biotin Supplements

Supplemental high-dose biotin can interfere with blood measurements (falsely high or low)!!

Will biotin help with hair loss and nails?

Nails- maybe.

Hair- No, hair loss mainly genetic.

Vitamin C History

• 400 BC- Hipocrates describes symptoms of scurvy.

• 1747- James Lind prescribes oranges and lemons as a cure for scurvy.

• 1932 Albert Szent-Gyorgyi identifies the antiscorbutic factor as “hexuronic acid,” a reducing agent.

• 1933- Chemical structure of Vitamin C was elucidated.

What is the chemical form of Vitamin C?

Ascorbic Acid → dissolves into liquids as Ascorbate (the form that is in blood).

Can humans synthesize Vitamin C?

Ascorbic Acid can be synthesized from glucose via Gulonolactone Oxidase, however humans and primates have a mutation in this enzyme and therefore cannot synthesize. All other animals can synthesize.

What are dietary sources of Vitamin C?

Citrus, red/orange veggies, cruciferous vegetables.

Ascorbic Acid is the predominant form of Vit. C in foods.

Ascorbic Acid oxidizes over time into Dehydroascorbic Acid. This form predominates in foods that have sat for a while after being picked.

Vit. C Absorption

Ascorbic Acid is absorbed by SVCT-1 (Sodium-Dependent Vitamin C Transporter) at the apical membrane and then SVCT-2 at the basal membrane.

Dehydroascorbic Acid is absorbed by GLUT at the apical membrane, converted back to Ascorbic Acid, and then into the capillaries at SVCT-2 at the basal membrane.

Interconversion of Ascorbic Acid and DHAA.

• AA to DHAA:

  • AA loses 1 electron to get to Semidehydroascorbic Acid Radical.

  • SAAR loses 1 electron to get to DHAA.

• DHAA to AA:

  • Dehydroascorbate Reductase catalyzes the reaction.

  • Glutathione acts as coenzyme.

Vitamin C Transporter

Ascorbate is main circulating form

Vit. C Tissue Uptake

Taken up by SVCT-1 and/or SVCT-2 in most tissues.

Vit. C Excretion

Excreted in urine as Ascorbate, DHAA, or Metabolites at consumption over 500mg.

Oxalic Acid: one of the metabolites of Vitamin C. A potentially dangerous side effect of high-dose vitamin C: kidney stones made of crystalized oxalic acid.

Vit.C Functions

• Functions as a reducing agent (donates electrons) in enzymatic reactions in the synthesis

  • Collagen

  • Carnitine

  • Neurotransmitters

  • Tyrosine

• Can also function nonenzymatically as a reducing agent as a soluble antioxidant.

Collagen Synthesis

• Requires the activity of several Vit. C dependent enzymes:

  • Prolyl 3-hydroxylase

  • Prolyl 4- hydroxylase

  • Lysyl hydroxylase

  • Add hydroxyl group to carbons!

• Active site of hydroxylases contain iron that donates an electron to the enzyme.

• Vitamin C then donates an electron to the oxidized iron (Fe3+) to get it back to it’s active, reduced state (Fe2+).

• Need proline to be hydroxylated to form triple helix structure of collagen.

Neurotransmitter Synthesis

In Dopamine Synthesis, Ascorbate donates an electron to Dopamine Beta-Hydroxylase to keep the iron in it’s active site in the reduced and active form.

Vitamin C as an Antioxidant

Unfortunately, metabolic pathways produce many Reactive Oxygen Species (ROS).

Vitamin C can donate electrons to ROS (Superoxide, Hydrogen Peroxide, Hydroxyl Radical) to stabilize them and prevent oxidative damage.

• Superoxide (O2) → H2O2

• Hydrogen Peroxide (H2O2) → 2 H2O

• Hydroxyl Radical (OH) → H2O

Assessing Vitamin C Status

• Plasma Vitamin C: more indicative to recent diet.

  • Normal Range: 0.6-0.8mg/dL; under 0.2 is deficient

• Leukocyte Vitamin C: indicative of tissue stores.

Vitamin C Deficiency

Scurvy!

Rare today but seen in malnourished individuals, alcoholics, the poor, and the elderly.

Vit. C Deficiency Clinical Symptoms

• Bleeding gums

• Petechiae (pinpoint hemorrhages)

• Easy bruising

• Joint pain

• Loose and decaying teeth

What is the RDA and UL for Vitamin C?

RDA: ~80 mg

Males (90mg), Women (75mg), Pregnancy, Lactation, and Smokers (oxidative damage) need more.

UL: 2g/day

Vit. C Toxicity

Up to 2g are routinely consumed without adverse effects.

With ~2g, most common side effect is abdominal pain and diarrhea.

Possible formation of kidney stones (oxalic acid).

Does Vitamin C actually help with the common cold?

It may have a modest benefit but the evidence is not overwhelming. It cannot hurt.

Linus Paulling- Vitamin C and the common cold!