unit 9 part 3A and 3B

how was niacin discovered 

– Discovered through the condition pellagra in humans and a similar condition, called black
tongue, in dogs
• Niacin was considered the “anti-black tongue” factor
– High incidence in areas where corn is the main dietary staple (because niacin is attached to
indigestible carbohydrates in corn, therefore it is poorly absorbed)

dietary sources for niacin

• fish

• meats

• bread

• cereal

• coffee

• tea 

niacin in coffee

trigonelline is converted to nicotinic acid by heat

in plant foods, how is vitamin B3 predominantly found

nicotinic acid, which is the provitamin

in animal derived foods, how is vitamin B3 commonly found

• nicotinamide

• nicotinamide adenine dinucleotide (NAD)

• nicotinamide adenine dinucleotide phosphate (NADP)

niacin in liver

Niacin can also be produced in the liver from the amino acid tryptophan (but only
about 1/60th of tryptophan is converted into nicotinamide)

digestion and absorption of NAD and NADP

• Digestion of NAD and NADP is required before absorption
– Hydrolyzed by the glycohydrolase enzyme to release free nicotinamide

nicotinic acid and nicotinamide absorption in the stomach

Nicotinic acid and nicotinamide are absorbed a bit in the stomach, but most is absorbed in the small intestine through sodium-dependent diffusion

niacin in plasma

In plasma, niacin circulates primarily as free nicotinamide

how can nicotinic acid and nicotinamide cross cell membranes

simple diffusion in most tissues, except kidney and RBC (cell-mediated)

nicotinic acid and nicotinamide are precursors for what in the body

NAD

what happens once NAD(P) is synthesized

niacin is trapped within the cell, in their reduced forms, the main functions are:

• NADH: transfer of electrons to ETC

• NADPH: reducing agent in biochemical pathways 

NAD(P) production

2 steps (from nicotinic acid):

1. convert acid to an amide

2. build into a dinucleotide structure 

NAD+ reduced to NADH in:

• glycolysis, kreb’s cycle, B-oxidation

• role in catabolism

NADP+ reduce to NADPH in blank and used for what

• hexose monophosphate shunt

• used for fatty acid synthesis, glutathione regeneration, etc

• role in anabolism

what is corn/maize

• Corn/Maize
– Contains significant amounts of niacin, but it’s bound and not absorbed
– Also deficient in Tryptophan
– Use of lime (from limestone, not the fruit) can help release niacin from corn
• Used by Indigenous peoples in food practices, but not in Africa

deficiency of niacin

• pellagra 

• 4 Ds: dermatitis, dementia, diarrhea, death

RDA for niacin

• includes niacin produced from tryptophan

• so we consider niacin equivalents (NE)

• NE= mg preformed niacin + mg trp/60

• 14/16mg per day NE adult women/men

sources of vitamin B2 (riboflavin)

• Rich in foods of animal origin
– Milk, milk products, meat, etc. (Source of free riboflavin)
– Other foods contain flavins, found as either flavin mononucleotide (FMN) or flavin adenine dinucleotide (FAD), which are essentially bound riboflavin
– Riboflavin is destroyed by sunlight (which is why milk is no longer sold in glass bottles)

absorption of riboflavin

• Riboflavin that is bound to proteins must be released prior to absorption
– This is done by HCl (protein denaturation) in the stomach
• Free riboflavin is absorbed from the gut lumen by an active transport mechanism known as the riboflavin transporter 2 (RFT2)

transport of riboflavin

Riboflavin, FAD, FMN are transported in the body bound to proteins (in particular albumin) 

storage of riboflavin

Riboflavin is stored a little bit in the body (the liver, kidney, and heart), but extra riboflavin is generally
excreted in urine
- Sufficient amounts in the body to last 2-6 weeks when riboflavin is no longer consumed in the diet

FMN and FAD

• FMN and FAD are made in cells:
– FMN and FAD are involved in redox reactions
– FAD is the primary form of riboflavin in the body (60-95%)
– Production is positively regulated by T3 hormone, which increases the activity of the flavokinase enzyme

ribotol and flavin

ribotol= linear ribose

flavin= nitrogenous base

what is FMN considered

nucleotide (base+sugar+P)

what is FAD considered

dinucleotide (FMN+adenine+sugar+P)

riboflavin biochemical pathways

• involved in many such as B-oxidation and krebs cycle

• delivers electrons to ETC

how do FMN and FAD function

• similar to NAD(P) in electron transfer

• difference is that FMN and FAD are typically bound to the active site of an enzyme

how does glutathione regenerate

FAD accepts 2 electrons from NADPH becoming FADH2

what is required to reform glutathione

niacin and riboflavin

RDA for riboflavin

• men= 1.3mg/day

• women= 1.1mg/day 

deficiency of riboflavin

• ariboflavinosis

• signs appear after ~3-4 months 

• deficiency relatively common when dietary intake is insufficient because riboflavin is continuously excreted in urine 

• no clear riboflavin deficiency has been characterized because typically occurs with other vitamin deficiencies, it is reversible 

• cracked and red lips, inflammation of lining of mouth and tongue, mouth ulcers, cracks at corner of mouth

what populations are at risk of riboflavin deficiency

Populations “at risk” for a deficiency?
– People with hypothyroidism or thyroid disease, because they
can’t make thyroid hormones to activate flavokinase
– Chronic alcoholism, which reduces riboflavin digestion &
absorption
– People who are lactose intolerant (reduced consumption of
milk)

UL for riboflavin

N/A

discovery of thiamine (vitamin B1)

• Need for thiamine first discovered in the 1800s when birds fed a diet of cooked polished rice developed neurological problems (now known as beriberi)
– Polished rice has had the husk, bran and germ removed

where is thiamine found

• many foods

• meats

• whole fortified or enriched grain products

how is thiamine destroyed

• heat or alkaline environments 

• cooking foods in hot water leads to loss of thiamine 

how does thiamine exist in plants

free form

how does thiamine exist in animals

• In animals, thiamine exists in phosphorylated form (known as thiamine pyrophosphate or TPP) – this is the active form
– Phosphate group must be removed to allow for its absorption

absorption of thiamine

• small intestine

• typically controlled by thiamine transporters

thiamine in plasma

• TPP found free form

• or bound to albumin

TPP is important in what key oxidative decarboxylation steps

1. pyruvate dehydrogenase complex

2. alpha-ketoglutarate dehydrogenase complex 

• These enzymes also require riboflavin (FAD), niacin (NAD), and pantothenic acid (CoA) as cofactors
• Critical in the movement of sugars and amino acids into energy metabolism pathways

TPP is also required in what

non-oxidative phase of the hexose monophosphate shunt (transketolase step), thus playing a role in the
synthesis of nucleotide precursors

RDA for thiamine

• men= 1.2mg/day

• women= 1.1mg/day

thiamine deficiency interferes with what

Thiamine deficiency interferes with critical energy metabolism pathways (pyruvate and alpha-ketoglutarate dehydrogenase complexes)
– Ultimately prevents ATP production and acetyl-CoA synthesis
– Causes an accumulation of pyruvate, lactate, and alpha-ketoglutarate in blood

thiamine deficiencies: dry beriberi

– Predominantly in adults due to chronic low thiamine intake
– Muscle weakness, affects the nervous system

thiamine deficiencies: wet beriberi

– Predominantly in children and young adults
– More severe than dry beriberi and affects the cardiovascular system

thiamine deficiencies: acute beriberi

– Occurs primarily in infants
– Anorexia, vomiting, lactic acidosis, and eventually death if left untreated

who is susceptible to thiamine deficiency

• chronic alcoholics

• people depending on polished (white) rice as a major source of food

UL for thiamine

N/A

pantothenic acid (vitamin B5) origin

• Derived from the Greek word pantos, which means “everywhere”
– Present everywhere (found in virtually all plant & animal foods); therefore deficiency is next to impossible
– Little to no toxicity associated with dietary and supplemental pantothenic acid

what form is pantotheic acid occurring as

• in foods both free and bound forms

• most pantothenic acid in food occurs as part of coenzyme A

absorption of pantothenic acid

in jejunum by passive diffusion

where is pantothenic acid found

free in blood

tissue uptake of pantothenic acid

Uptake into tissues occurs through the sodium-dependent multivitamin transporter (SMVT)

what is pantothenic acid a precursor for

synthesis of coenzyme A

what is pantothenic acid essential in

energy metabolism (formation of acetyl CoA) allowing krebs cycle to take place

active group of pantothenic acid

SH from cysteine

what is 4-phosphopantetheine

• active form in fatty acid synthesis 

• important component of acyl-carrier protein in the fatty acid synthase complex

what is coenzyme A (CoA, CoASH)

• active form in oxidative reactions

• energy production

• formation of acetyl-CoA

how was biotin (vitamin B7) discovered

Discovered in 1931 during experiments examining the cause of “egg white injury”
– Eating raw eggs led to hair loss, dermatitis, etc. This led to the discovery of avidin, a biotin-binding protein.

sources of biotin

1. made by intestinal bacteria

2. widely found in foods bound to proteins

absorption of biotin

must be fully removed from proteins prior to absorption

• proteolysis by pepsin in the stomach breaks down proteins and releases biotin 

• free biotin is absorbed to near completion 

what inhibits biotin absorbtion

alcohol

circulation of biotin

• circulates in the blood free form (~80%), with a bit bound to albumin

RDA for biotin

N/A or deficiency established because its so prevalent

3 key reactions involving biotin

1. pyruvate carboxylation (production of oxaloacetate) 

2. malonyl CoA formation

3. conversion of propionate into glucose (important in ruminants)

what requires a biotin cofactor to be active

pyruvate carboxylase

biotin function

• The biotin is anchored to a lysine side chain, allowing it to “reach” into the carboxylation reactive site (active site #1), where it is carboxylated

• The cofactor can then swing to the active site #2, bringing the carboxyl group into close proximity with the substrate (in this example, pyruvate)

recall what these are: NAD, NADP, FAD, TPP, pantothenic acid, biotin

• NAD= niacin (B3)

• NADP= niacin (B3)

• FAD= riboflavin (B2)

• TPP= thiamin (B1)

• pantothenic acid= CoA

• biotin= (B7)

how was folate (vitamin B9) discovered

• folate and vit B12 were discovered during the search to cure megaloblastic anemia, a problem first found in the 1800s

what does folate mean

Folate is a generic term that refers to both natural folates in food and the synthetic form used in supplements and fortified foods called folic acid

folic acid

Folic acid refers to the oxidized form of the vitamin found in fortified foods and supplements (100% bioavailable)

folate

Folate refers to the reduced form of the vitamin found naturally in foods (~50% bioavailable)

components of folate

• pterin ring; PABA (para-aminobenzoic acid); glutamic acid

• humans can synthesize all 3 but lack the enzymes to conjugate them 

• all 3 parts must be present for vitamin activity 

why is folate critical

• critical for production of nucleic acid precursors and several amino acids, as well as in methylation reactions 

• therefore folate is important in bone marrow and developing fetus 

absorption of folate

Natural folates have multiple glutamate residues (i.e., polyglutamates) which must be removed for absorption in its simplest form (monoglutamate)
– Polyglutamate hydrolase removes the glutamate residues
• This enzyme is sensitive to alcohol and inhibitors naturally present in certain foods (legumes, lentils, etc)
– Folic acid is already a monoglutamate structure, so no digestion is required

absorption of folate in the small intestine

• absorbed by proton-coupled folate transporter (PCFT)

absorption of folate in the intestine

• most natural folates/folic acid are converted into the bioactive compound 5-methyltetrahydrofolate 

folate in plasma

• you primarily detect 5-methyl THF and a bit of folate

• generally transported in blood bound to proteins, such as albumin

what is the bioactive form of folate

5-methyl THF

folate vs folic acid

• folate= multiple glutamate residues (pteroylpolyglutamate)

• folic acid= only 1 glutamate residue (pteroylmonoglutamate)

total DFE (dietary folate equivalent)=

ug food folate + (1.7 x ug folic acid)

what was the last vitamin to be discovered

vit B12 (cobalamin)

what is cobalamin a generic term for

• group of compounds called corrinoids

• because a corrin nucleus that contains cobalt, which is very rare

what produces vit B12

only bacteria

what contains vit B12

• animal products such as meat, poultry, eggs

• strict vegetarians/vegans are at risk of deficiency

what does vit B12 deficiency cause

• functional deficiency for folate (the two work together)

• neurological problems

RDA and UL for vit B12

• RDA= 2.5 ug/day

• UL= n/a

absorption of vit B12

• In the stomach, a specific binding protein is secreted from the gastric lining, known as intrinsic factor (IF)
• The Vit B12-IF complex goes to a receptor in the small intestine
• Complex broken down in the enterocyte. Vit B12 is absorbed, while IF is released back into the intestinal lumen
• Vit B12 is stored in the liver and can undergo enterohepatic circulation

two causes that lead to a vit B12 deficiency

1. not enough in your diet

2. improper absorption due to defects in intrinsic factor

what does folate deficiency lead to

• impaired DNA synthesis and repair

• uracil misincorporation

• megaloblastic anemia

the folate trap

• Only reaction that can metabolize N5-methyl THF is methionine synthase (Vit B12 dependent)
• When someone is Vit B12 deficient, N5-methyl THF “trapped” and can’t become THF
• Large doses of folate can overcome a Vit B12 deficiency. 5× RDA for folate saturates ability to form N5- methyl THF, so you get more folate going to the liver where it is converted into THF (this will bypass the trap)
• However, there is still a problem with the SAM cycle!

folate deficiency impact on fetus

• folate is important in dividing cells, which supports production of specialized cells that form the neural tube

• this takes place during early pregnancy, so folate deficiency leads to severe birth defects (neural tube defects; NTD)

• in the embryo, the neural tube is the CNS, which becomes the brain and spinal cord

• NTD is an opening of the spinal cord and brain

• can cause spina bifida, anencephaly, or encephalocele

discovery of vit B6

• found while trying to understand dermatitis in rats

forms of vit B6

6 vitamers which can be interchanged:

• pyridoxine (plant provitamin), pyridoxal, pyridoxamine

• all can be found phosphorylated and unphosphorylated

where are vit B6 isomers found

in food so deficiency in humans is very rare

absorption of vit B6

• dephosphorylated prior to absorption

• passive diffusion in the jejunum

main form of vit B6

PLP (pyridoxal phosphate) is the main form found in blood, bound to albumin

where are high levels of vit B6 found

muscle