Biochem of Micronutrients Exam 1 (Thiamin, Niacin, Riboflavin, Biotin, Vit B6)

What started the history of thiamin? When was it discovered?

End of 1800's in Asia neurodegenerative disease that caused muscle wasting and heart failure - beri beri
Anti-beri beri factor discovered in late 1800s - chicken experiment
Thiamin 1st synthesized early 1900s
1st B vitamin discovered

Thiamin: Major fxns

Energy metabolism - dehydrogenases (TCA cycle, pentose phosphate pathway, BCAA catabolism) Production of sugars for DNA/RNA - transketolases (nucleotides, NADPH synthesis) Production of neurotransmitters (promotes nerve impulses, myelin sheath maintenance)

Thiamin - stability

vulnerable to heat, water, and oxidation

What diseases result from thiamin deficiency?

Beri beri, Wernicke-Korsakoff

Thiamin- toxicity

None

Thiamin - DGA's

1.1 mg/day female. 1.2 mg/day male
Based on depletion/ repletion of erythrocyte transketolase (thiamin dependent enzyme)
No UL
Avg intake: 1.76mg/day - deficiency rare

Thiamin - dietary sources
Form in plants v animals

Found in all food groups, enriched/fortified grains
Phosphorylated form (TPP & TDP) in animal foods, Free form in plant foods - equally bioavailable
Synthesized from gut bacteria - absorption unknown

Thiamine Antagonists

Betel nut - degrades thiamine
Antithiamine compounds - inactivate thiamin via oxidation (ferns, betel nut, tea)
Thiaminases - rapidly degrade thiamin (raw fish/shellfish, ferns, microorganisms)

Thiamine - Digestion

Dephosphorylated by intestinal phosphatases and pyrophosphatases
TPP (animal) > Thiamin (plant and dephosphorylated animal) > transported via THTR1 or THTR2 into enterocyte > transported via THTR1 into circulation

Thiamine - Absorption

At brush border - active or passive (saturable - low concentrations are carrier mediated (THTR1 & THTR2) and high concentrations are passive diffusion)
Only absorb 4.5mg for doses >30mg

What increases and decreases thiamine absorption?

Increase: deficiency, decrease extracellular thiamin concentration, increased expression of THTR2 but not THTR1
Decrease: ethanol (alcohol reduces expression of transporters). infection, GI disease

Thiamine - Transport (forms of thiamine in cell v in blood)

By portal blood to liver, circulates in blood/ enters cells as free thiamine or TMP, phosphorylated to TPP in cells (active form - not in blood not good indicator of status)

Thiamine - Storage

Roughly 30 mg in body (2-3 wks worth, need roughly 1mg/day)
Small concentrations stored in liver, muscle, heart, kidney, brain

Thiamine - Excretion

Via urine & sweat, half life of 1-12 hrs, excreted as intact thiamine

Thiamine Dependent Enzymes and cycle they're involved in

Pyruvate Dehydrogenase - TCA cycle (mito)
A ketoglutarate dehydrogenase - TCA cycle (mito)
BCKDH - BCAA metabolism (mito)
Transketolase - Pentose phosphate pathway (cyt)

Thiamin Dependent Enzymes and they're rxns

Pyruvate Dehydrogenase - pyruvate > acetyl coA (TCA)
A ketoglutarate dehydrogenase - A ketoglutarate > succinyl coA (TCA)
BCKDH - A ketoglutarate > Acyl CoA (BCAA metabolism)
Transketolase - ribose 5 phosphate > glyceraldehyde 3 phosphate (pentose phosphate pathway)
All enzymes require niacin (NAD) and riboflavin (FAD)

Thiamin - Deficiency Consequences

Energy failure (A ketoglutarate dehydrogenase) - reduction in TCA, not making ATP
Accumulation of lactate (pyruvate dehydrogenase) - acidemia, edema in brain
Reduced neurotransmitter synthesis (BCKDH) - acetylcholine, glutamate, GABA

Thiamin - risk factors for deficiency

Inadequate intake/absorption - refined grain, alcoholism, malnutrition, gastric bypass, PN
Increased loss - V/D, diuretics
Increased utilization - Pregnancy/lactation, refeeding syndrome

What is wet beri beri?

Cardiovascular - medical emergency
S/S - tachycardia, cardiomegaly, edema, heart failure
Result of extreme physical exertion and high carb intake

What is dry beri beri?

More common than wet
S/S: anorexia, weakness, ataxia, mental impairment
Peripheral neuropathy ("burning feet syndrome", impaired sensory, symmetrical motor, and reflex fxn
Often see w/ inactivity and calorie restriction

What is wernicke-korsakoff?

Part of dry beri beri - Alcohol abuse (80-90% cases)
Stage 1: Wernickes encephalopathy - acute brain disorder (reversible)
Stage 2: Korsakoff syndrome - chronic memory disorder (irreversible)
S/S: Mental- confusion, trouble concentrating, dorwsiness, loss of memory, hallucinations. Ataxia, Vision changes

Thiamine - Deficiency treatment

IV thiamine, oral, replete Mg (needed to convert thiamine to TPP) and folate (needed to regenerate TPP)

Niacin - History

Early 1900's american south - disease causing scaley mouth, mouth sores, diarrhea, confusion, mental deterioration
120,000 died from 1900-1920
1940's identify as pellegra, niacin added to enriched grains
Prison experiment - originally thought be infectious but prisoners on poor diet would get and not guards

Niacin - Fxn

Energy metabolism - coenzyme for e- transfer in hundreds of redox rxns (precursor for NAD)
DNA synthesis and repair - required substrate
Synthesis of glucose, FA, cholesterol and steroid hormones
NAD used in cellular respiration, ketone pathways, AA, FA, DNA repair (catabolic and anabolic rxns)

Niacin - Stability

Minimal vulnerability to heat, air, and light

Niacin - Toxicity

UL - 35 mg/d, large doses can cause liver damage
Usually occurs from supplements
S/S: vasodialtory flushing, heartburn, N/V, hepatoxicity, hyperuricemia & gout (competes w/ uric acid for excretion), insulin resistance/ glucose intolerance

Niacin - DGAs

2-8 mg/d kids, 14 mg/d women, 16 mg/d men (AI for kids, RDA for adults)
Estimated intake corresponds with N1-methylnicotinamide (above minimal excretion = pellagra occurs)
RDA in is in Niacin equivalents (NE)

Niacin - Dietary sources

In plant and animal foods, enriched grains
Can be synthesized from tyrptophan (60 mg trypt = 1 mg niacin)
Most dietary niacin is bound to proteins rendering it unavailable
Alkaline (base) unbinds protein from Niacin - soaking corn in lime increases bioavailability

Niacin - Digestion

None

Niacin - Absorption

Stomach: passive diffusion (saturable)
Sm int: Na+ dependent facilitated diffusion and carrier mediated active transport

Niacin - Metabolism

Niacin/ Nicotinamide - converted to NMN then to NAD
Tryptophan can be converted to NAD (kyneurine pathway)

Niacin - Storage

None

Niacin - Excretion

Via urine

Niacin - Redox rxns

OIL (oxidation is loss) RIG (reduction is gain) of e-
NADH (reduced), NADPH (phosphorylated reduced), NAD+ (oxidized), NADP+ (oxidized phosphorylated)

What are sirtuins?

NAD dependent enzyme that removes acetyl group
In brain (increase glucose tolerance/food intake), Fat (increase lipogenesis, decrease adipogenesis), Pancreas (increase insulin secretion), Muscle (Increase FA oxidation/insulin action), liver (Increase lipolysis/GNG)

What are histones?

Structural support of chromosomes, important for gene regulation/ expression (one of SiRTs target proteins)

Niacin - Mechanism

NAD+ activates histones using SiRT to remove acetyl group (reduces inflammation, DNA repair, tumor suppression, mitochondrial biogenesis) then NAD+ becomes NAM which is converted to NMN by NAMPT, NMN converted back to NAD+ (salvage pathway regenerates NAD+)

What impacts SiRTs?

Aging - decreases NAMPT d/t disruption of sleep (Build up of NAM). Inhibition of SiRT (stop build up of NAM), Decrease salvage pathway = decrease NAD+ = Decrease in metabolic fxn
Calorie restriction - increase salvage pathway = Increase SiRTs. Decrease glucose, Increased respiration/NAD/NADH/SiRTs. Counteracts effect of aging

Niacin - Cancer

Preventative - DNA repair, cancer cells use NAD and ATP metabolism (primarily glycolysis), inhibition of NAD pathway kills cancer, chemotherapy can target NAD synthesis
Less free radicals d/t Increased mitochondrial regeneration

Niacin - pharmacological use

Treatment of hyperlipidemia - decrease Tg and LDL, inhibits catabolism of HDL

Niacin - Deficiency risk factors

Poor diet (rare), diet with low bioavailability from CHO, malabsorption/alcoholism, disorders of tryptophan

Niacin - Deficiency

Pellagra: 4 D's - dermatitis, diarrhea, dementia, death
Leading cause of death in mental hospitals
Dermatitis: In sun exposed areas of skin, sun is lower in urocanic acid - requires Niacin for UV light absorption

Riboflavin - History

1870s - discovered yellow pigment in milk
1930s - B complex recognized as having 2 parts: thiamin (heat liable) and riboflavin (heat stable) & riboflavin characterized

Riboflavin - Fxns

E- carrier in energy metabolism - metabolism of macros via TCA and ETC
Activation of antioxidants - redox rxns
Metabolism of other B vits
Shuttles e- through ETC to ATP synthase
FA to acetyl coA

Riboflavin - Toxicity

None

Riboflavin - Deficiency

Ariboflavinosis - glossitis (loss of taste buds, inflamed tongue), sore throat, cheilosis, stomatitis, seborrheic dermatitis
Occurs frequently with other B vit deficiency.
Increases incidence of preeclampsia

Riboflavin - DGAs

AI infants: 0.3-0.4 mg/d
RDA: 1.1 mg/d women, 1.3 mg men
Usual intake: 1-3.6 mg/d - deficiency rare
No UL

Riboflavin - Dietary sources

Widely distributed - Milk products! (1/3-1/2 riboflavin from milk) Enriched grains
Mostly bound to protein in food
Some bacteria production - unknown utilization
95% bioavailble

Riboflavin - Digestion

Digested from protein binding (gastric acid, luminal proteases, brush border phosphatases) - decreased digestion in elderly d/t decreased stomach acid

Riboflavin - Absorption

Active transport dependent transporters (RFT/RFVT) up to 25 mg
Passive diffusion (saturable)

Riboflavin - Transport

Riboflavin (50%), FMN (40%), FAD (19%)
Mostly protein bound
RFVT - riboflavin specific transporter that brings from lumen to blood

Riboflavin - Storage

Greatest in liver, kidney, heart
Once in cell phosphorylated to FMN & FAD
Storage of 2-6 wks

Riboflavin - Excretion

Via urine

Riboflavin - Mechanism

Glutathione Redox cycle
Glutathione neutralizes free radical by donating e-, becomes oxidized (uses glutathione peroxidase & selenium). FAD donate e- in order to regenerate reduced glutathione so it can be an antioxidant again (uses NAD and glutathione reductase)
Xanthine oxidase: creates uric acid (antioxidant)

Riboflavin - Metabolism of B vits

B6: Activation requires an FMN-dependent enzyme
Niacin: conversion of tryptophan to NAD requires FAD dependent enzyme
Folate: FAD dependent enzyme involved in 1 C cycling

Riboflavin - Deficiency Risk Factors

Alcoholics (decreased intake/absorption)
Anorexia
Lactose intolerance
Increased need (high PA, pregnancy, lactation, adrenal insufficiency, hypothyroidism)
Subclinical deficiency - inflammation

B6 - history

1950s - unfortified formula, some infants developed seizures, alleviated by B6 supplementation

B6 - 6 forms
Plant v Animal forms

PN, PNP - alcohol
PM, PMP - amine
PL, PLP - aldehyde *active form

B6 - food sources

Fortified cereals, F & V, protein
Plant - PNP, PN
Animal = PMP, PMP, PL, PLP
Some bacterial synthesis and absorption in colon

B6 - RDA

0.1 mg - 6 months
0.3-1.3 mg - Kids
1.3 mg - adults
1.5-1.7 mg - 51+

B6- Digestion

Dephosphorylated to enter enterocyte from lumen
Phosphorylated in enterocyte
Dephosphorylated to leave cell into portal vein
Phosphorylation keeps in cell - regulates concentration
PMP/PNP converted to PLP then PL - starts in enterocyte but all converted to PL in liver

B6 - Absorption

Carrier mediated
Passive diffusion
Bioavailability = 75%
W/in cell and liver it's phosphorylated

B6 - Transport

In liver converted to PL or PLP
In circulation its bound to a protein
Extrahepatic uptake as PL/PLP only

B6 - Storage

Stored in muscle (75%), liver, brain, kidney, spleen

B6- Excretion

Catabolism - PL oxidized to pyridoxic acid (PA)
Urinary excretion
Short term marker of intake (PA)

B6 - Function

Acts as a coenzyme (PLP) in gluconeogenesis and AA metabolism (1 C transfer)
Metabolizes tryptophan to niacin
Helps in production of Hgb and some neurotransmitters

B6 - Mechanism (3 rxns)

Decarboxylase coenzyme - removes CO2 to produce amines
Histidine > Histamine (allergic rxns)
Tryptophan > serotonin (happiness)
Homocysteine > cysteine (1 C metabolism)

B6 - tryptophan metabolism in depth

Tryptophan > hyneurine . 3 OH-kyneurine (needs B6) > 3 OH xanthurenic acid > niacin
Tryptophan loading test indicate B6 status - build of 3 OH xanthurenic acid bc can't be converted to niacin

B6 - Assessment

Plasma PLP
Erythrocyte transaminase (B6 as coenzyme)
Urinary 4 - pyridoxic acid excretion
tryptophan loading (urinary excretion of xanthurenic acid)

B6 - Deficiency

Rare
At risk: alcoholism, certain meds
Symptoms: rash, glossitis, neurologic (confusion, neuropathy, seizures)

B6 - Toxicity

Only due to supplementation (12 mg/d)
Peripheral nueropathy

Biotin - History

1920s - Noticed egg whites caused toxic symptoms
1930/40 - Isolated and structure discovered
1980s - stopped being recommended for hair loss

Biotin - DGAs

Infants 5-6 ug, Children 8-20 ug, Adults - 30ug, Breastfeeding - 35 ug
No UL

Biotin - Food Sources

Egg yolk. liver, yeast
Synthesized by gut bacteria (absorption unknown)

Biotin - Digestion

Biotinidase frees biotin from proteins to be absorbed by sm int

Biotin - Absorption

From sm int to portal vein via hSMVT (competes w/ B6)

Biotin - Transport

Freely transported by blood

Biotin - Storage

Insignificant amounts in muscles, liver, brain

Biotin - Excretion

Via urine

Biotin - Interactions / Bioavailabilty

Competes with B6 and lipoic acid for hSMVT, anticonvulsant inhibit absorption
Intereferes with certain labs (troponin, reproductive/thyroid meds)
100% bioavailable
heat stable

Biotin - Fxns

Biotinylation of carboxylases - Binds as coenzyme activating (carboxylases add Co2 needed for cellular res)
Biotin recycling - Biotinidase converts back to biotin by removing AA
FA/AA metabolism - activates carboxylases needed to produce acetyl CoA and Succinyl CoA

Biotin - Mechanism

Biotin dependent enzyme required for conversion of acetyl coA to malonyl coA.
W/ ATP converted by ACC1 and malonyl coA does FA synthesis.
W/o ATP converted by ACC2 which inhibits CPT1 (responsible for letting acyl coA into mitochondria to undergo beta oxidation to become acetyl coA) - stop FA synthesis and promotes FA oxidation

Biotin - Risk for deficiency

PN, babies on formula, raw egg whites, smoking ,pregnancy, liver disease, anticonvulsant meds

Biotin - Deficiency

Very rare
Hair loss, scaly rash around eyes, nose, mouth, genitals, nerologic (depression, lethargy, hallucinations, peripheral neuropathy, ataxia, seizures)
Biotin deficient facies - facial rash with abnormal facial fat distribution
Impaired immune system/ Increased susceptibility to infection

Biotin - Diabetes

Deficiency could be associated with impaired glucose utilization, hypoglycemia and abnormal regulation. Supplementation may decrease FBG - more research needed

Biotin - Hair/nails

Could help control hair in kids with uncombable hair syndrome - no evidence to prevent hair loss

Digestion Synopsis

Thiamin - Dephosphorylated
Niacin - None
Riboflavin - Removed from protein binding (via HCl)
Biotin - Removed from proteins (via biotinidase)
B6- Dephosphorylated to enter/leave cell, phosphrylated to stay in cell PMP/PM/PNP/ PN > PLP/PL

Absorption Synopsis

Thiamin - THTR1 & THTR2
Niacin - Saturable (active/passive)
Riboflavin - RFT/RFVT saturable >25mg (active to passive)
Biotin - hSMVT (competes w/ B6)
B6 - saturable (active/passive)

Transport Synopsis

Thiamin - As free form, phosphorylated in cells
Niacin - Carrier mediated active transport
Riboflavin - Mostly protein bound
Biotin - Freely transported
B6 - In liver converted to PLP/PL, bound to protein in circulation

Storage Synopsis

Thiamin - 30 mg (2-3 wks worth in liver,muscle,etc)
Niacin - None
Riboflavin - 2-6 wks as FMN or FAD in liver, kidney, heart
Biotin - Insignificant amount
B6 - Stored in muscle, liver, brain, kidneys, spleen

Excretion Synopsis

Thiamin - Urine as intact thiamine, half life 1-12hrs
Niacin - via urine
Riboflavin - via urine
Biotin - Via urine
B6 - Via urine as pyridoxic acid

Toxicity Synopsis

Thiamine - No UL
Niacin - 30 mg/day (flushing, N/V, hepatotoxicity)
Riboflavin - No UL
Biotin - No UL
B6 - 12 mg/day (peripheral neuropathy)

Deficiency Synopsis

Thiamin - Beri Beri (wet/dry), Wernicke-Korsakoff
Niacin - Pellagra (4 D's)
Riboflavin - Ariboflavinosis
Biotin - hair loss, rash, Biotin deficient facies
B6 - Rash, glossitis, neurologic

Mechanism Synopsis

Thiamin - Dehydrogenase/ transketolase rxns
Niacin - SiRTs/ Salvage pathway
Riboflavin - Glutathione redox cycle
Biotin - Activation of carboxylases in FA synthesis
B6 - Tryptophan > Niacin

Enzyme/Coenzyme (thiamin, biotin, B6)

Thiamin - dehydrogenases and transketolases
Biotin - carboxylases
B6- decarboxylases