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Vitamins B5, B6, B7

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Pantothenic Acid (B5)

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  • coenzyme form is coenzyme A (CoA or CoASH due to presence of SH group, which is also the reaction site)

  • many reactions

    • SCoA high energy bond - adds energy to reactions​

  • widespread in food (whole grains, cereals, meats, legumes)

    • often in conjunction with other conditions such as DM, alcoholism, IBD​

  • essential in a variety of reactions that sustain life

    • required from chemical reactions that generate energy from food​

    • synthesis of essential fats, CHL and steroid hormones, acetylcholine, and melatonin

    • heme requires a CoA-containing compound for synthesis

    • metabolism of a number of drugs and toxins by the liver requires CoA

Pantothenic Acid Deficiency

  • isolated pantothenic acid deficiency is very rare in humans

  • ​observed during severe malnutrition
  • symptoms: 
    • burning feet syndrome​
      • numbness of toes​
      • burning sensation in toes
    • vomiting
    • fatigue
    • irritability
    • weakness
  • symptoms often difficult to assess since they are subtle and resemble those of other B vitamin deficiencies​​

Pantothenic Acid Absorption

  • CoA hydrolyzed in the intestinal lumen to dephospho-CoA, phosphopantetheine, and pantetheine

    • pantetheine sebsequently hydrolyzed to pantothenic acid​

    • pantothenic acid only form absorbed

      • Na+ linked active transport mechanism: sodium linked multivitamin transporter (SMVT)​​​

        • antiporter - proton pump; saturable at high intake levels​

      • some passive diffusion but only at high intake levels

  • bioavailability poorly researched or understood; assumed approx. 40-60 absorbed

  • may be synthesizes by intestinal microbes, <1% of human requirements

Pantothenic Acid Transport, Assimilation and Assessment

  • plasma - carries free acid form

  • erythrocytes

    • absorb pantothenic acid by diffusion​

    • carry out most of the vitamin in the blood

  • assimilation

    • Na+ linked co-transport mechanism​

    • hormones influence

    • converted to CoA in tissue - predominant tissue form

    • found in liver, adrenals, kidneys, brain, heart and testes

    • tissue levels not affected by dietary deprivation

    • CSF needs constant supply - acetylcholine synthesis in brain

  • plasma concentration not a good indicator

    • —Blood levels of 1.6-2.7 uM

    • —Pantothenic acid is excreted intact in urine (reliable indicator),

    • —Measured with a Lactobacillus plantarum assay or a radioimmunoassay.

    • —Amount excreted varies proportionally with dietary intake over a wide range of intake

    • —Urinary excretion < 1 mg/day   ~    poor status

Clinical Application

  • Reduced serum CHL

    • High doses (500-1200 mg/day)

    • Mechanism unclear

  • —Rheumatoid Arthritis (RA)

    • —​2g/d reduces stiffness

  • —Athletic performance

    • —Reduced lactic acid accumulation

    • —Data questionable

  • —Wound healing

    • —Increases recruitment of fibroblasts, increases skin hydration

Pyridoxine (B6)

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B6 Food Sources

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  • fortified cereal is a major source

  • pyridoxine hydrochloride in supplements

  • hind gut microbe's can produce 86% of human needs

  • variable content in food and instability in heat, light, and under alkaline conditions make bioavailability highly variable (0-70%)

  • storage loss is an issue

Digestion, Absorption and Storage

Absorption

  • has to be dephosphorylated before absorption

    • alkaline phosphatase, Zn dependent​

    • products: PN, PM, PL

  • absorption occurs primarily in jejunum by passive diffusion

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Assimilation

  • liver is the main organ that takes up and metabolizes newly absorbed (dephosphorylated) vitamin B6

    • need to be phosphorylated first​

    • most is transported in plasma in PLP form bound to albumin/proteins

    • in RBC, bound to hemoglobin (6x the amount in plasma)

    • PL crosses cell membranes more readily than PLP

study question: 

  • why does B6 need to be dephosphorylated just to be phosphorylated again?

Activation

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In the liver: 

  • PN, PL, and PM are converted to PNP, PLP, and PMP by PL kinase

  • PNP, which is normally found only at very low concentrations, and PMP are oxidized to PLP by PNP oxidase.

  • PMP is also generated from PLP by aminotransferase reactions (see transamination reactions next).

  • PLP is bound to various proteins in tissues; this protects it from the action of phosphatases.

B6 Functions

Pyridoxal phosphate (PLP) is a coenzyme for many enzymes (>140) involved in amino acid metabolism

  • —Transamination, transulfuration, single carbon metabolism, niacin synthesis, gluconeogenesis, hemoglobin production, and the synthesis of neurotransmitters as well as histimine

  • —Activity based on the aldehyde form’s ability to interact with primary amino groups

  • —PN is easily oxidized under mild oxidizing conditions to yield PL (aldehyde form)

  • B6 is involved in homocysteine metabolism

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1. Transamination

  • AST

  • ALT

2. Decarboxylation

  • formation of GABA, serotonin, dopamine, etc

3. Glycogen degradation

  • PLP is required for glycogen phosphorylase

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B6 Deficiency

  • rare in the US

  • populations at risk: 

    • elderly​

    • alcoholics

    • high protein intake (increased requirement)

    • certain drugs

B6 Toxicity

  • toxicity has only been documented for vitamin B6 from high doses of supplements, not from food

  • long term use of high dose (>200mg/day) can cause sensory neuropathy

    • pain, numbness of extremities​

    • difficulty walking in severe cases

  • UL: 100mg/day

  • <200mg/day seems to be safe

B6 Clinical Application

  • homocysteine and cardiovascular diseases

  • large doses to treat certain diseases are not well supported by currently available evidence

B6 Assessment

  • plasma PLP concentrations are thought to be the best indicator of status

  • —<20 nmol/L   = Deficient

  • —20~30 nmol/L =  Marginal

  • —>30 nmol/L   = Adequate

Biotin (B7)

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B7 Sources

  • widley distributed

  • good food sources

    • liver, egg yolk​

    • legumes, soybean, nuts, cereal

    • brewer's yeast

  • significant synthesis by bacteria in the colon; 5% of daily human need

B7 Functions

  • biotin is required by carboxylases to add carbon in energy pathways

  • carboxylation: a reaction that adds carbon from CO2 to the carbon chain

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B7 Absorption

  • Biotin exists as free biotin and in protein-bound forms in foods.

  • Little is known about factors that affect bioavailability. Less than ½ of biotin in whole food is thought to be bioavailable.

    • The hydrolysis of protein-bound biotin has not been well characterized

    • Most dietary biotin is protein bound in both meats and cereals; biotin in cereals appears to be less bioavailable.

  • Avidin, a protein found in raw egg white, binds biotin in the small intestine and prevent its absorption

  • A biotin carrier (SMVT) in the intestinal brush border membrane transports biotin against a sodium ion concentration gradient; at high concentrations, diffusion predominates

Absorption

  • Biotin exists as free biotin and in protein-bound forms in foods.

  • Little is known about factors that affect bioavailability. Less than ½ of biotin in whole food is thought to be bioavailable.

    • The hydrolysis of protein-bound biotin has not been well characterized

    • Most dietary biotin is protein bound in both meats and cereals; biotin in cereals appears to be less bioavailable.

  • Avidin, a protein found in raw egg white, binds biotin in the small intestine and prevent its absorption

  • A biotin carrier (SMVT) in the intestinal brush border membrane transports biotin against a sodium ion concentration gradient; at high concentrations, diffusion predominates

B7 Deficiency

  • —Increased need in pregnancy and lactation (as many as 1/3 of pregnancies may be subclinical deficient)

  • —Consumption of raw eggs for a long time

    • —The protein avidin  in egg white binds to biotin -prevents absorption

    • —Strongest known non-covalent bond in nature

  • —Diseases that cause impaired biotin absorption

    • —e.g., inflammatory bowel disease

  • —Alcoholism

  • —Athletes

  • —Burn patients

  • —Elderly

  • —Symptoms

    • —Hair loss, a scaly red rash around mouth, eyes, and genital area

    • —Neurological: Lethargy, Depression, Hallucination

B7 Assessment

  • —Plasma concentration 215-750 pg/mL

    • —Low level not a good indicator for intake or status

  • —Urinary excretion

    • —Decreased excretion of biotin 

    • —Increased excretion of metabolites

  • —All recent studies on biotin nutriture have used one of three basic types of assays to estimate biotin: bioassays (most studies), avidin-binding assays, or fluorescent derivative assays.

©2023 by Syracuse University Dr.Margaret Voss

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