V6 vitaminb6

Vitamin B6

Omar A Obeid

NFSC 315

Vitamin B6

• Gyorgy 1934: Antidermatitis (Acrodynia) factor in rats.

• Crystallized (1938) by 3 groups

• Synthesized (1939) in USA and Germany

• 1945: Three forms of the vitamin

VITAMIN B6

PYRIDOXAMINE PYRIDOXAL

PYRIDOXINE

NH2

N

OHHO

N

OHHO

N

OHHO

HO

OH

Vitamin B6

• Exists as several vitamers– Interchangeable– Pyridoxine (PN): alcohol (OH) form– Pyridoxal (PL): aldehyde form – Pyridoxamine (PM): amine (NH2) form

• Each has a 5’-phosphate derivative– PNP, PLP, PMP– Function as coenzymes

NH

CH2OH

OH

CH3

HOH2C

Pyridoxine (PN)

+NH

C

OH

CH3

HOH2C

Pyridoxal (PL)

+

HO

Pyridoxamine Phosphate (PMP)

NH

CH2

OH

CH3+

NH2

O-

-O P O H2C

O

NH

C

OH

CH3

Pyridoxal phosphate (PLP)

+

HO

O-

-O P O H2C

O

NH

CH2

OH

CH3

HOH2C

Pyridoxamine (PM)

+

NH2

NH

CH2OH

OH

CH3

Pyridoxine phosphate (PNP)

O-

-O P O H2C

O

+

Vitamin B6 Structures

Chemistry

• Colorless crystalline solid

• Soluble in water and alcohol

• Stable to heat (-al and -amine less)

• Decompose in alkali and light

• Pyridoxal 5-phosphate and pyridoxamine 5-phosphate are the coenzyme forms.

Sources

• All vitamers are found in food• Pyridoxine

– Most stable– Found mainly in plants – Bananas, navy beans, walnuts

• Pyridoxamine and Pyridoxal– Found in animal products– Sirloin stead, salmon, and light meat of

chicken.

Vitamin B6 Content of Foods

Food Vitamin B6(mg/100g) Glycosylated(%) Dairy products Milk 0.04 _ Yogurt 0.05 _ Cheeses 0.04-0.08 _

Meat Beef 0.33 _ Chicken 0.33-0.68 _ Lamb 0.28 _ Pork 0.35 _ Ham 0.32 _ Calf liver 0.84 _ Herring 0.37 _ Haddock 0.18 _ Tuna 0.43 _ Oysters 0.05 _ Shrimp 0.10 _


Food Vitamin B6(mg/100g) Glycosylated(%)

Cereals Corn meal 0.20 _ Rice Polished 0.17 20 Unpolished 0.55 23 Wheat, whole 0.29 28

Other Eggs 0.19 _ Human colostrum 0.001-0.002 _ Human milk 0.010-0.025 _



Vegetables Asparagus 0.15 _ Beans 0.08-0.18 _ Broccoli 0.17 66 Brussels sprouts 0.18 _ Cabbage 0.16 46 Carrots 0.15 51-86 Cauliflower 0.21 66 Celery 0.06 _ Corn 0.20 _ Onions 0.13 _ Peas 0.16 15 Potatoes 0.25 32 Spinach 0.28 50



Fruits Apples 0.03 _ Grapefruit 0.03 _ Oranges 0.06 _ Peaches 0.02 _ Strawberries 0.06 _ Tomatoes 0.10 46

NUTS Peanuts 0.40 _ Pecans 0.18 _ Walnuts 0.73 7

Sources: USDA data; Leklem, J.E. (1996). In: “Present Understanding in Nutrition” (E.E. Ziegler and L.J. Filter,Jr.,eds.), 7th Ed., p.175. ISLI Press, Washington, D.C.

Concentrations (µg per 100g)and proportions of vitamin B6 derivatives in selected foods.

Total B6 %P %Free %PNG

VEGETABLES Carrots 206 18 29 54

FRUITSApple raw 104 33 52 15 Apple juice 87 26 65 9Orange 83 39 33 28Orange juice 55 42 36 23

CEREALSWhole wheat bread 79 39 33 51 White bread 16 42 51 8

NUTS/SEEDSAlmonds 137 5 95 0Sunflower seeds 605 14 34 52Soyabean 267 34 49 18

Sources: Reynolds,1998; Bitsh and Schramm, 1992.%P=%Phosphorylated vitamers; %Free=%non-phosphorylated vitamers; %PNG=%PN glucosides.

Sources

• Loss during refining of cereals (flour)

• Large % Vitamin B6 can bound to proteins via amino or sulfhydryl groups.– Less available: resistant to hydrolysis and low

B6 activity.

• Vitamin B6 react with glucose and become glycosylated.– Less available

• Storage: loss 10-50%

Vitamin B6• Digestion

– Phosphorylated vitamers must be dephosphorylated prior to absorption

• Intestinal phosphatases

• Absorption– PN, PL and PM absorbed primarily by non-saturable

process (passive diffusion)• Absorption of dietary B6 ranges from 71-82%

• Diffusion linked to phosphorylation: Jejunum and ileum.

• Dephosphorylation by membrane bound alkaline phosphatase

Absorption of the B6 Vitamins

Pyridoxamine PM PMPPyridoxal PL PLP B6Pyridoxine PN PNP

ATP

ADPPi

Mucosal Cell

Vitamin B6

• Within Enterocyte– PN phosphorylated to PNP

• Pyridoxine kinase (ATP, Zn)

– PL phosphorylated to PLP• Kinase (ATP, Zn)

– PNP may be converted to PLP• Pyridoxine phosphate oxidase (FMN)

• Blood– PLP is main form (~60%) of vitamin in blood

• PL also exists • Both PL and PLP are bound to albumin

– 0.1% of total B6 in plasma as PLP, bound to protein

Vitamin B6

• Erythrocytes– PL and PN (rapid simple diffusion) PLP

– Tight binding to Hb (role in transport!)

• Liver– Stores about 5 to 10% or vitamin

– Phosphorylation occurs within cytoplasm– PNP and PMP are converted to PLP– PL and PLP are released into blood for

transport to extrahepatic tissues

Vitamin B6

• Requires removal of P by phosphatase to enter cells

• Muscles (possess majority of PLP)– PLP must first be hydrolyzed to PL before uptake– Within cell rephosphorylated– 80-90% in muscle bound to glycogen

phosphorylase

– Decrease with low energy intake

• B6 in body mainly as PLP• Human body store is 40-150mg, sufficient for

20-75 days

Metabolism• Interconversion of vatimers• Metabolism mainly in liver

– PMP and PNP are converted to PLP by:• Pyridoxal phosphate oxidase require FMN

– Dephosphrylation of PLP to PL to 4-PA

• Intracellular level of PLP – Controlled by enzymatic hydrolysis

• Excess PLP will be hydrolyzed to PL

– Controlled by product inhibition of PNP/PMP oxidase

Metabolism/Excretion

• Excess PL is converted to Pyridoxic acid (PA)– PA excreted in urine– PA excretion reflects recent vitamin intake

• Newly formed PLP is not freely exchangeable with endogenous PLP

• Major product for excretion is 4-pyridoxic acid• Urinary 4-pyridoxic acid is inversely related to

protein intake• Interacts with folate and B12

Metabolic functions

• Amino acid metabolism

• Gluconeogenesis/glycogen catabolism

• Niacin formation

• Nervous system

• Immune function

• Hormone modulation

• Lipid metabolism

Functions

• Amino Acid Metabolism– PLP via formation of a Schiff base labilizes all the

bonds around the alpha carbon of the amino acid

– Schiff base• Product formed by an amino group and an aldehyde

– The specific bond that is broken is determined by the enzyme

• Decarboxylase, Transaminases, Aldolases

Transaminations

• PLP and PMP serves as coenzymes– Aspartic amino transferase (AST)

• Aspartate donates its amino group to an alpha keto acid forming OAA and a different amino acid

– Alanine aminotransferase (ALT)• Alanine donates its amine group to an alpha keto

acid forming pyruvate and a different amino acid

Transaminations

• Phase I– The corresponding alpha keto acid (pyruvate)

of the amino acid (alanine) is produced along with PMP

• Phase II– New alpha keto acid (alpha keto glutarate)

receives amino group from PMP producing the new amino acid (glutamate) and PLP

Decarboxylations

• GABA Synthesis – Glutamate decarboxylase– Conversion of Glutamate to GABA

• Serotonin Production – 5-Hydroxytryptophan decarboxylase– Conversion of 5-hydroxytryptophan to

serotonin (5-hydroxytryptamine)

Metabolic interconversions of the B6 vitamers

PLPPNP PMP

PNPL PM

4-PA

PNPoxidase (FMN)

PNPoxidase (FMN)

Transaminase

Aldehyde

oxidase (F

AD

)

Aldehyde

dehydrogenase (N

AD

)

Pase

Pase

Pase

PLkinase

PLkinase

PLkinase

Transulfhydrations and Desulfhydrations

• PLP required for cysteine synthesis from methionine– Both cystathionine beta synthase (CBS) and

cystathionase require PLP

• PLP required for desulfhydration followed by transamination to generate pyruvate.

Other reactions

• Cleavage– PLP required for removal of the methyl group from

serine and transfer to THF• Glycine produced as well

• Racemization– PLP required by racemases that catalyze

interconversion of D- and L- amino acids

• Synthesis– PLP necessary for synthesis of heme, niacin,

histamine from histidine, carnitine, taurine, dopamine and more.

Effect of Protein Intake on Vitamin B6 Status

TreatmentProtein intake (g/Kg/d): 0.5 1.0 2.0

Vitamin B6 intake (mg/g protein): 0.04 0.02 0.01

Parameter (adequate value) Percentage of subjects with low values

Urinary 4-pyridoxic acid(>3 mmol/day) 11 22 78

Urinary total vitamin B6(>0.5 mmol/day) 56 56 67

Plasma pyridoxal phosphate(>30 mmol/liter) 33 67 78

Urinary xanthurenic acid(<65 mmol/day) 11 11 44

Source: Hansen, C. M., Leklem, J.E., and Miller, L.T. (1996). J. Nutr. 126, 1891-1901.

Gluconeogenesis/glycogen catabolism

• Transamination and glycogen phosphorylase • Vitamin B6 deficient rats

– Low liver and muscle glycogen phosphorylase

– No effect on B6 conc in Muscle, unlike calorie restriction

• Rats: IV. B6 (300mg/kg)– Low liver glycogen and high plasma glucose

• In human:– No clear relation

Nervous system

• Neurotransmitters: serotonin, taurine, dopamine, norepinephrine, histamine, GABA

• Rats: mother deficient in B6:– Offspring: Brain abnormalities

• Infant fed formula low in B6– Abnormal electroencephalograms (EEGs)

• Adults with B6 deficiency• Abnormal EEGs with high protein diet

B6 and synthesis of neutransmitters

Glutamate γ – aminobutyric acid (GABA)Glutamatedecarboxylase

Tryptophan 5 hydroxy tryp 5-OH-Tryptamine(Serotonin)PLP

O2 CO2

Hydroxylase

Tyrosine DOPA Dopamine

Norepinephrine

Epinephrine

O2CO2

O2

Hydroxylase

SAMCH3

Immune system

• serine transhydroymethylase (PLP) for 1C metabolism (nucleic acid synthesis) immune function

• Vitamin B6 deficient animals:– Low lymphocyte production, antibody response to

antigens, cell mediated immunity

• Human:– Elderly with impaired immune system: respond to

50mg PN/d – Young: Marginal deficiency : no effect– Health elderly: relation between B6 and immunity (IL-

2)

Effect of Vitamin B6 Status on Mitogenic Responses and Interleukin 2 Production by Peripheral Blood

Mononucleocytes of Elderly Humans

Parameter Baseline B6 deprived B6 supplemented

Mitogenic response Concanavalin A 120 70 190 Phytohemagglutinin 100 70 100 Staphylococcus aureas 115 60 200

IL-2 production (Ku/Liter) 105 40 145

Source: Meydani, S.N., Ribaya-Meradi, J.D., Russel, R.M., Sahyoung, N., Morrow, F.D., and Gershoff, S.N. (1991). Am. J. Clin. Nutr. 53, 1275-1280 .

Erythrocyte function

• Binding of PL to α-chain Hb increases O2 binding affinity

• Binding of PLP to β-chain Hb decreases O2 binding affinity

• PLP cofactor for δ-aminolevulinic acid synthetase (heme synthesis)

• B6 deficiency:– Microcytic anemia– Pyridoxine responsive anemia

Hormone modulation/gene expression

• Reversible reaction with receptors for:– Estrogen, androgen, progesterone, glucocorticoid at

lysine residues.

• Vitamin B6 deficient rats:– 3H-estradiol: more incorporation at uterine tissues– Zn and B6 deficiency: more interaction– No of estrogen receptors not affected– mRNA albumin increased (7 times)– mRNA of cytosolic aminotransferase (7 times)

• Vitamin B6 may be a modulator of gene expression

Lipid metabolism

• Similarity between EFA and B6 deficiencies

• B6 deficient rats: low body fat

• Conversion of linoleic to arachidonic

• Arachidonic acid and cholesterol

• PLP for carnitine synthesis– Clarification is needed

Cellular processes affected by PLP

Cellular process Function

1-C metabolism, hormone modulation Immune function

Glycogen phosphorylase, transamination

Gluconeogenesis

Tryptophan metabolism Niacin formation

Heme synthesis, transamination, O2 affinity

Red cell metabolism and formation

Neurotransmitter synthesis, lipid metabolism

Nervous system

Hormone modulation, binding of PLP to lysine on hormone receptors

Hormone modulation

Nutrient Interactions

• Vitamin B6 is interrelated with Riboflavin– Riboflavin is coenzyme of PNP/PMP oxidase

which converts PNP/PMP to PLP

• Vitamin B6 is interrelated with Niacin– Niacin is coenzyme for aldehyde

dehydrogenase which oxidizes PL to PA. – Conversion of tryptophan to niacin

Drug-vitamin B6 interactionDrug Examples Mechanism of interactions

Hydrazines Iproniazid, isoniazid, hydralazine

Reacts with Pl and PLP to forms a hydrazone

Antibiotic cycloserine Reacts with PLP to form an oxime

L-DOPA L-3,4-(HO)2-phenylalanine

Reacts with PLP to form tetrahydroquinoline derivatives

Chelator Penicillamine Reacts with PLP to form thiazolidine

Oral contraceptives

Increase enzyme level in liver and other tissues, retention of PLP

Alcohol Ethanol Increased catabolism of PLP

Vitamin B6 and disease

• Coronary heart disease– Altered S-AA metabolism– Hcy elevation– Cystathionine β-synthase deficiency:

• Arteriosclerosis

– PLP and atherosclerosis: independent of Hcy and cholesterol

– Relation with cholesterol– Immunity


• HIV/AIDS– B6 and progression: inverse– Low status

– PLP binds to CD4 receptors

– PLP in a noncompetitive inhibitor of HIV-1 reverse transscriptase

• Premenstrual syndrome– PLP status similar between PMS and non-symptoms– B6 suppl: improvement in some symptoms

– 150-200mg!!

– Cell transport competition, receptor


• Sickle cell anemia– Low level– 100mg PN-HCl (2m): low severity, frequency and

duration of painful crisis– PL and PLP binding to Hb

• Asthma– Low PLP status– 100mg: PN-HCl: low duration, occurrence and

severity– Theophylline: Low plasma and RBC PLP

• Carpal tunnel syndrome– Most studies: PN suppl relif symptoms of pain and

numbness in hands

Vitamin B6: High doses

• Is toxic in pharmacological amounts• Chronic ingestion of 2-6 g pyridoxine/d may

cause sensory neuropathy– Signs similar to deficiency

• Has been used to treat a variety of conditions – Atherosclerosis, carpal tunnel syndrome,

premenstrual syndrome, depression, muscular fatigue.

• Rats (500-100mg/kg)– Decrease in testis epididymis, prostate gland,

mature spermatid counts

Dietary Reference Intakes (DRI) For Vitamin B6

Females Males

Life stage group RDA (mg/d) RDA (mg/d)/ U L** (mg/d)

Infants

0- 6 months 0.1* 0.1* ND

7-12 months 0.3* 0.3* ND

Children

1- 3 yrs. 0.5 0.5 30

4- 8 yrs. 0.6 0.6 40

9-13 yrs. 1.0 1.0 60

14-18 yrs. 1.2 1.3 80

19-50 yrs. 1.3 1.3 100

> 50 yrs. 1.5 1.7 100

Pregnancy

< 18 yrs. 1.9 - 80

> 18 yrs. 1.9 - 100

Lactation

< 18 yrs. 2.0 - 80

> 18 yrs 2.0 - 100From Institute of Medicine, Food and Nutrition Board. Dietary Reference Intakes for Thiamin, Riboflavin, Niacin, Vitamin B6, Folate, Vitamin B12, Pantothenic Acid, Biotin and Choline. Washington, DC: National Academy Press, 1998.

Vitamin B6

• 1989 RDA=1.6 - 2.0 mg/d

– Based on protein intake

• 1998 RDA– Adult = 1.3 mg/d

• UL:– Adult: 100mg/d

Deficiency

• Rare

• Sign of deficiency can be quickly corrected by administration.

• Deficiency Signs– Fatigue, cheilosis, glossitis, seizures,

convulsions in infants, hypochromic, microcytic anemia (impaired heme synthesis)

Pyridoxine (Vitamin B6)

Pyridoxine deficiency:The scaling seborrhea-like dermatosis in this patient was induced in volunteers by giving the pyridoxine antagonist desoxypyridoxine. Such lesions have not been proved to occur spontaneously although it is suspected that some instances are due to pyridoxine deficiency.

Pyridoxine (Vitamin B6)

Pyridoxine deficiency:The glossitis in this patient was induced in volunteers by giving the pyridoxine antagonist desoxypyridoxine. This is undistinguishable from that due to deficiency of other B group vitamins.

•Breastfed infants born with low Vitamin B6•Elderly

•Poor intake and possibly accelerated hydrolysis of PLP and oxidation of PL to PIC

•Alcoholics•Conversion of PN and PM to PLP impaired

•Persons on maintenance dialysis•Variety of Drug Therapies

•Isoniazid, corticosteroids, anticonvulsants

Groups at Risk for Deficiency

Assessment

• Direct:– Plasma PLP(>30nM)– Plasma total B6 (>40nM)– Urinary 4-pyridoxic acid (>3.0µmol/day)

• Indirect:– Tryptophan loading (2g): xanturenic acid– Methionine loading (3g): Hcy, cystathionine– Erythrocyte transaminase stimulation

• Dietary intake– Intake, B6:protein ratio (>0.02), PN-β-glucosidase

• EEG pattern

Assessment of Status(Tryptophan Load)

TRYPTOPHAN

N-FORMYLKYNURENINE

KYNURENINE

3-OH-KYNURENINE

3-OH ANTHRANILIC ACID

QUINOLINIC ACID

NIACIN

Kynureninase (PLP)XanthurenicAcid acetyl

CoA

acetoacetylCoA

Assessment of Status

• Erythrocyte Transaminase Index– Erythrocyte alanine aminotransferase– Erythrocyte aspartate aminotransferase

– Look at the activity of the enzyme before and after addition of Vitamin B6

– A two-fold or more increase in activity of the enzyme after addition of vitamin B6 is indicative of deficiency

– Less than a two-fold increase in activity is indicative of acceptable status

V6 vitaminb6

Health & Medicine

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