1. Vitamin B6 Metabolism
Nishita Mohan
BBS – G1

2. Index What is Vitamin B6? Discovery Role in Metabolism Deficiency
Treatment
Future directions

3. ???????

4. What is Vitamin B6?
Vitamin B6 is a water-soluble vitamin that exists in three main forms: pyridoxine (PN), pyridoxal (PL), and pyridoxamine (PM)
Active form of vitamin B6- pyridoxal 5'-phosphate(PLP) serves as a coenzyme in many reactions in amino acid, glucose, and lipid metabolism
Pyridoxamine and pyridoxal are labile compounds which are rapidly destroyed on exposure to air, heat, or light.
B6 originally present in foods can be lost in processing: prolonged heating, cooking, sterilization and freezing.

5. The vitamers

6. 1930s: Rudolf Peters and the curious case of rat acrodynia
Discovery of Vitamin B6
1930s: Rudolf Peters and the curious case of rat acrodynia

7. 1930s: Rudolf Peters and the curious case of rat acrodynia
Discovery of Vitamin B6
1930s: Rudolf Peters and the curious case of rat acrodynia
1934: Paul György showed that the factor which cured 'rat acrodynia' was vitamin B(6)

8. 1930s: Rudolf Peters and the curious case of rat acrodynia
Discovery of Vitamin B6
1930s: Rudolf Peters and the curious case of rat acrodynia
1934: Paul György showed that the factor which cured 'rat acrodynia' was vitamin B(6)
Richard Kuhn and his associates independently showed that vitamin B(6) was a pyridine derivative, 3-hydroxy-4,5-dihydroxy-methyl-2-methyl-pyridine. 

9. Average adult needs 1.5-1.7 mg of vitamin B6!
Sources of Vitamin B6
Average adult needs mg of vitamin B6!

10. Absorption of vitamin B6
Passive diffusion of ‘free forms’ in the jejunum and iluem
Membrane bound alkaline phosphatases required for absorption in the duodenum
Vitamin B6 is absorbed in the small intestine, mostly in the free forms after the glycoside or phosphate groups are removed. As mentioned above, a portion of PNG is absorbed intact, and when consumed in high concentrations, the phosphorylated forms can also be absorbed intact. After absorption, some of the free forms are taken up by the red blood cells and bound to hemoglobin,9 but the majority of B6 goes to the liver for conversion into the active form, pyridoxal-5-phosphate, or PLP (also called P- 5-P). PLP is then released into the bloodstream, traveling to the rest of the body largely bound to the protein albumin. A small amount of PLP is retained by the liver for storage.9 Finally, B6 enters the target tissues after removal of the phosphate group from the circulating PLP; muscle tissues store the majority of the body’s B6.9 Because most vitamin B6 metabolism occurs in the liver, individuals with liver disease are at high risk for B6 deficiency.2

11. Absorption of vitamin B6
Passive diffusion of ‘free forms’ in the jejunum and iluem
Membrane bound alkaline phosphatases required for absorption in the duodenum
Vitamin B6 is absorbed in the small intestine, mostly in the free forms after the glycoside or phosphate groups are removed. As mentioned above, a portion of PNG is absorbed intact, and when consumed in high concentrations, the phosphorylated forms can also be absorbed intact. After absorption, some of the free forms are taken up by the red blood cells and bound to hemoglobin,9 but the majority of B6 goes to the liver for conversion into the active form, pyridoxal-5-phosphate, or PLP (also called P- 5-P). PLP is then released into the bloodstream, traveling to the rest of the body largely bound to the protein albumin. A small amount of PLP is retained by the liver for storage.9 Finally, B6 enters the target tissues after removal of the phosphate group from the circulating PLP; muscle tissues store the majority of the body’s B6.9 Because most vitamin B6 metabolism occurs in the liver, individuals with liver disease are at high risk for B6 deficiency.2
Converted into the active form, pyridoxal-5-phosphate (PLP) by Pyridoxal Kinase (PDXK)

12. Absorption of vitamin B6: Overview
Passive diffusion of ‘free forms’ in the jejunum and iluem
PLP released into the blood stream and travels to tissues bound to Albumin
Membrane bound alkaline phosphatases required for absorption in the duodenum
Vitamin B6 is absorbed in the small intestine, mostly in the free forms after the glycoside or phosphate groups are removed. As mentioned above, a portion of PNG is absorbed intact, and when consumed in high concentrations, the phosphorylated forms can also be absorbed intact. After absorption, some of the free forms are taken up by the red blood cells and bound to hemoglobin,9 but the majority of B6 goes to the liver for conversion into the active form, pyridoxal-5-phosphate, or PLP (also called P- 5-P). PLP is then released into the bloodstream, traveling to the rest of the body largely bound to the protein albumin. A small amount of PLP is retained by the liver for storage.9 Finally, B6 enters the target tissues after removal of the phosphate group from the circulating PLP; muscle tissues store the majority of the body’s B6.9 Because most vitamin B6 metabolism occurs in the liver, individuals with liver disease are at high risk for B6 deficiency.2
Converted into the active form, pyridoxal-5-phosphate (PLP) by Pyridoxal Kinase (PDXK)

13. Fate of B6: A closer look AOX1 PDXP PDXP/ PDXK PDXP/ PDXK PDXP

14. Intestine: PDXP (Phosphatase)
Fate of B6: A closer look
AOX1
Of course, don’t panic!
Intestine: PDXP (Phosphatase)
Liver: PDXP and PDXK (Both kinase and phosphatase) Blood: Major form: PLP
Excreted form: 4-PA
PDXP
PDXP/ PDXK
PDXP/ PDXK
PDXP
PDXP/ PDXK

15. Distribution of Vitamin B6
High turnover!
A large amount is excreted out
Majority (70-8-%) of the body’s vitamin B6 is stored in the muscle
10% Small amount of PLP retained
 Finally, B6 enters the target tissues after removal of the phosphate group from the circulating PLP; muscle tissues store the majority of the body’s B6.9 Because most vitamin B6 metabolism occurs in the liver, individuals with liver disease are at high risk for B6 deficiency.26
It is important to emphasize the fact that the absorption and utilization of B6 can vary greatly among individuals.8 There are a few factors that likely explain this variability. First, the activity of intestinal enzymes that split off glucosides from PNG (plant form) in order to facilitate absorption has been shown to vary with the intake of PNG.27 Second, the intestinal absorption of both PNP and PMP (phosphorylated animal forms), as well as the final tissue uptake of circulating PLP requires a different enzyme that has zinc as a co-factor. Third, riboflavin, or vitamin B2, is needed for the phosphorylation of all forms of absorbed B6 to active PLP in the liver. Thus, poor intakes of zinc and/or riboflavin may exacerbate or lead to B6 deficiencies. Diets that restrict meat, seafood, poultry, dairy or eggs, or diets that largely rely on improperly prepared grains, legumes and nuts that are high in mineral-binding phytates present a three-fold problem: they are low in riboflavin, low in bioavailable zinc, and they contain vitamin B6 mainly in a form that is more difficult for the body to absorb and utilize.

16. Should I really study this vitamin?!
Amino acid biosynthesis & degradation
Sphingolipid biosynthesis
Heme biosynthesis
Neurotransmitter biosynthesis &degradation
Glycogen metabolism
Folate biosynthesis

17. Types of reactions catalyzed
Transamination
Decarboxylation
C-C/O bond cleavage
Racemization

18. Types of reactions catalyzed
Transamination
Decarboxylation
C-C/O bond cleavage
Racemization

19. Types of reactions catalyzed
Transamination:
α-ketoglutarate
glutamate
Hydroxyphenyl-pyruvic Acid
Tyrosine
Tyrosine Transaminase, catalysed by PLP
Where did I see this before???

20. Phenylalanine Metabolism
Kiana’s talk: Aromatic amino acid metabolism
α-ketoglutarate
glutamate
Phenylalanine hydroxylase
Tyrosine Transaminase
Hydroxyphenyl-pyruvic Acid
Phenylalanine
Tyrosine
Hydroxyphenyl-pyruvate dioxygenase
4-Mayleylacetoacetate isomerase
Homogentisate Oxidase
4-Fumarylacetoacetic Acid
4-Maleylacetoacetate
Homogentisic Acid
4-Fumarylacetoacetase
Fumarate
Acetoacetate
Adapted from American College of Medical Genetic snad Genomics: PKU

21. Types of reactions catalysed
Decarboxylation
Songlei’s talk

22. Important properties of PLP
2 important chemical properties of PLP:
Ability to form imines through the aldehyde group
Nitrogen can function as a strongly electron withdrawing group- ‘electron sink’

23. Role in Glycogen Metabolism
????
Dr. Michel’s lecture

24. Role in Glucose Metabolism
Dr. Michel’s lecture

25. Role in Glucose Metabolism
1. Formation of an [E-PL · Pi · glycogen] ternary complex. 2. Cleavage of the glycosidic bond by an acid catalysis (the Pi donates H to the bridge O because the Pi receives H from PL), and formation of oxonium ion intermediate (half-chair conformation). 3. Reaction of Pi with the oxonium ion to form G1P.
Proton donor
Proton Acceptor
NOTE: Phosphate group of PLP plays the important role!!

26. What kind of a reaction is this?
Transamination
Decarboxylation
Heterolytic bond cleavage
Racemization

27. What kind of a reaction is this?
Transamination
Decarboxylation
Heterolytic bond cleavage
Racemization

28. Role in amino acid metabolism
Imine formation
Aldehyde group plays the key role!!
Function: Electron sink
The aldehyde group of PLP can form an aldimine, or Schiff base, with the α amino group of the substrate, which releases water.*
After binding of the substrate to PLP, a catalytic base in the active side abstracts its α hydrogen as a proton, and the surplus electron left behind travels all the way down to the nitrogen at the bottom of the PLP ring, inverting the entire sequence of single and double bonds along the way. This has the effect of turning the bond between the α carbon and the α nitrogen into a Schiff base, which then undergoes hydrolysis to release pyruvate.
Inwha’s talk

29. Role in amino acid metabolism
Pyruvate
Fate??

30. Role in amino acid metabolism
NOTICE HOW THE CATALYSIS MECHANISMS ARE COMPLETELY DIFFERENT!!

31. What kind of a reaction is this?
Transamination
Decarboxylation
Heterolytic bond cleavage
Racemization

32. What kind of a reaction is this?
Transamination
Decarboxylation
Heterolytic bond cleavage
Racemization

33. Neurotransmitter biosynthesis
Dan’s talk

34. Clinical correlation
Isolated vitamin B6 deficiency is uncommon; associated with
low concentrations of other B group vitamins –B12 & folate
microcytic anemia
electroencephalographic abnormalities
dermatitis and glossitis (swollen tongue)
depression and confusion
weakened immune function
Individuals with borderline vitamin B6 concentrations or mild deficiency might have no deficiency signs or symptoms for months or even years.

35. Clinical correlation
Microcytic anemia
Cofactor: PLP

36. Risk factors Impaired renal function
Malabsorptive autoimmune disorders- celiac disease, Crohn's disease, ulcerative colitis, inflammatory bowel disease 
Alcohol dependence
Patients with celiac disease, Crohn's disease, ulcerative colitis, inflammatory bowel disease, and other malabsorptive autoimmune disorders tend to have low plasma PLP concentrations. The mechanisms for this effect are not known. However, celiac disease is associated with lower pyridoxine absorption, and low PLP concentrations in inflammatory bowel disease could be due to the inflammatory response.
Alcohol produces acetaldehyde, which decreases net PLP formation by cells and competes with PLP in protein binding [1,3]. As a result, the PLP in cells might be more susceptible to hydrolysis by membrane-bound phosphatase.  

37. Future directions
Potential role in lowering the risk of cardiovascular diseases
Vitamin B6 and the immune system
Role in the reduction of PMS symptoms, morning sickness
Interactions with different medication and nutrients

38. Summary
Vitamin B6 is an important co-factor required in several metabolic pathways
It is readily absorbed in the small intestine and the uptake is mediated by passive diffusion
It has several interconvertible forms but the active form is PLP
Enables the breakdown of glycogen to glucose, amino acid metabolism, serotonin production
Deficiency of vitamin b6 is associated with certain groups which have high risk factors

39. THANK YOU!