Vitamin B Lecture 2
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1. What is the best source of B6? (2) Why?
2. How is it found in foods? (2)
3. WHere is it found in rice?
4. What conditions destabalize it? 3
5. Most stable form? What is it used for?
- 1. Raw vegetables and nuts, b/c cooking destroys over 50% of B6.
- 2. In free form and protein bound
- 3. Aleuronic layer (polished rice doesn't have this)
- 4. Best conditions are acidic - so neutral/alkalinic conditions, heat, and light.
- 5. Pyridoxine is most stable, so used in fortification and supplements.
1. What are the four (five) forms of B6?
2. Which is active form? 1
3. Which are reduced forms? 2
- 1. Pyridoxal, pyridoxal phosphate, pyridoxamine (phosphate), pyridoxine
- 2. Pyridoxal phosphate.
- 3. Pyridoxal and pyridoxal phosphate
What are B6's 3 functions in AA metabolism?
1. Pyridoxal phosphate-dep transaminases catabolize most AAs
2. Transulfuration for methionine --> homocysteine --> cysteine
3. cofactor for kynureninase: tryptophan --> niacin
1. What is B6's role with tryptophan? What happens in B6 def in this rxn?
2. B6's role in lipid metabolism?
3. B6's role in NT synthesis? (2 enzymes, 4 NTs)
- 1. Cofactor for kynureninase - w/o B6, tryptophan is diverted to another rxn and is lost in urine.
- 2. Sphingolipid synthesis
- 3. (1) cofactor for tyrosine decarboxylase - NE, Epinephrine, and serotonin.
- (2) cofactor for gluatamate decarboxylase (GABA)
1. What is B6's role in GNG? (2)
2. Role in immune system?
3. How does B6 affect O2 transport? In this regard, what can B6 be used as treatment for?
4. What else does B6 affect?
- 1. Transamination in AA catabolism for substrates; coenzyme of glycogen phosphorylase to form G1P for release into blood.
- 2. Histamine synthesis
- 3. Heme synthesis - increases binding affinity of O2 for Hb.
Can be used as treatment for sickle cell anemia.
4. Gene expression
1. Where does absorption of B6 occur?
2. What is bioavailability like? What would limit absorption? (2)
3. Describe mech? (2)
4. Main form in blood? How does it travel in blood?
- 1. Jejunum and ileum
- 2. Bioavailability is high (70-80%) unless high amount in blood or bound to proteins
- 3. (1) P'lated forms are dephosphorylated by alkaline phosphatase (2) absorbed via passive diffusion
- 4. PLP - albumin.
1. After uptake, what happens to B6?
2. How is B6 found in tissue? (in what form & bound to what or free?) Ex of muscle?
- 1. B6 is phosphorylated into predominant form by pyridoxal kinase into PLP
- 2. Found as PLP(predominant form) and bound to enzymes. In muscle, it's bound to glycogen phosphorylase.
1. What is major storage site of B6?
2. Central organ for metabolism of B6? What occurs here?
3. What are key enzymes in each organ? (1,5)
- 1. Muscle
- 2. Liver - interconversion of B6 - mostly to PLP
- 3. Glycogen phosphorylase in muscle and in liver:
- - Pyridoxal kinase: pyrixodine, pyridoxal and pyridoxamine --> p'lated versions
- - Alkaline phosphatase: dephosphorylates p'lated versions (esp in gut!)
- - Pyridoxamine phosphate oxidase: catalyzes rate limiting step in B6 metabolism
- - Pyridoxal dehydrogenase: reduced forms can be oxidized by this enzyme to yield pyridoxal or pyridoxal phosphate.
- - Transaminases
1. What happens in deficiency? (3)
2. Symptoms? Name 3 of 6
- 1. Homocysteinemia, affects postnatal development of glutamate receptor, important for learning, colon cancer.
- 2. Seborrheic dermatitis, cracks in mouth, intertrigo (inflammation of body folds), drowsiness, neuropathy, conjunctivitis
- 3. None
1. How is biotin always found?
2. Sources? (4)
3. What is it destabilized by?
4. Is meat a good source? Is corn?
- 1. Covalently bound to lysl group of biotin-dep enzyme
- 2. Milk, liver, egg yolk, raw veggies
- 3. Heat
- 4. No, corn is (100%), but other grains can be 20%
1. What is biotin's main, overall function?
2. What two enzymes need it most?
3. How does B7 affect epigenetics?
- 1. Transfer of covalently ound, single-C units in CO2 form (carboxylation)
- 2. Pyruvate carboxylase (GNG): pyruvate --> OAA; AcCoA carboxylase (FA synthesis) catalyzes first committed step (Acyl CoA --> Malonyl CoA)
- 3. Biotinylation of H4 increases length of DNA wrapped around histone --> DECREASES TRANSCRIPTION
Describe absorption mechanism of B7 (3)
What does this have in common with B5?
Describe uptake transporters (2)
- 1. Intestial proteases: protein-bound biotin --> biocytin
- 2. Biotinidase: biocytin --> free biotin
- 3. Free biotin is absorbed via active transport (Na+dep multivitamin transporter - SMVT) and by passive diffusion at high concentrations.
Uptake transporters: SMVT and MCT1 (Monocarboxylate transporter1)
B5 is also absorbed with SMVT.
1. How is biotin stored? (tissue and cell organelle)
2. Describe progression of deficiency illness (4)
3. What are the causes of deficiency? (3)
- 1. Mostly in liver in mitochondria as part of acetyl CoA carboxylase (ACC)
- 2. Hair loss/rashes --> convulsions & neurological disorders --> changes in blood pH --> coma/death
- 3. Genetic (can't free biotin in stomach), lots of raw egg whites (avidin), usually seen in breastfed infants with B7 def moms.
1. What is general significance of B5?
2. What form is it found in? What foods? what is it susceptible to? Bioavailability?
3. Describe mech of absorption? (4) - two options for uptake into enterocytes.
4. What form is it transported in? What is it bound to?
- 1. Forming CoA and ACP - forms high-energy thioester bonds for metabolism of macronutrients
- 2. Bound (ACP or CoA) - meats, mushrooms, avocado. Nothing - fairly stable. BIoavailability unknown
- 3. (1) Pancreatic enzymes free B5 from CoA and ACP (2) 4PPT is dephosphorylated to form pantetheine (3) Pantetheinase: pantetheine --> pantothenic acid (4) Pantothenic acid is taken up by SVMT or in high concentrations, by passive diffusion
- 4. Free form, but mostly bound to RBCs (~90%)
1. What is cellular uptake for B5?
2. Where is B5 stored? 5 organs, 1 organelle
3. When does def occur? (2)
4. Name symptoms (4)
- 1. SMVT
- 2. Liver, adrenals, kidney, brain, heart - mitochondria.
- 3. Only in severely malnourished people or people taking B5 antagonists
- 4. Paresthesia, depression, fatigue, muscle weakness, GI problems
1. Which cells can create CoA? From what?
2. Why are tissue levels not susceptible to B5 deficiency?
- 1. All, from dietary pantothenic acid
- 2. Because it's efficiently conserved and recycled.
1. What is B12 a general descriptor for?
2. What rxns does it play important role in?
3. Who can only make B12? So how are B12 vitamins found in food?
4. What are the two active forms? What is the form found in supplements?
- 1. Corrinoids - compounds containing cobalt-centered corrin nucleus
- 2. One C metabolism
- 3. Bacteria- by bacterially fermented foods or in tissues obtained from ruminal/intestinal microflora
- 4. Methylcobalamin and 5'deoxyadenysylcobalamin; cyanocobalamin
1. What are the two main functions of B12? Name accompanying enzymes.
- 1. Formation of methionine from homocysteine (methionine synthase + B12 - homocysteine --> methionine)
- 2. Methylmalonyl CoA mutase (methylmalonyl CoA to succinyl CoA) in degradation of propionate formed from catabolism of odd chain FAs (B6 also plays role in odd chain FA catabolism)
1. Draw B12 and methionine synthase, draw enzymes and other necessary vitamins (6 substrates) two vitamins, 1 enzyme
1. How do Fe and vitamin C affect uptake of B12? B9?
2. What protein is necessary for B12 uptake? Why? (2)
3. Describe methods of transport from lumen into cells (2)
- 1. Oxidize B12 into less desirable form. Increases B9 absorption
- 2. Intrinsic factor (released from parietal cells) b/c it protects vitamin from hydrolytic attack and bacterial catabolism
- 3. Active transport via IF receptor that transports IF-B12 complex OR SIMPLE DIFFUSION in high doses for old people
1. Name two transport proteins in blood for vitamin B12 - which is storage? Which binds more B12 and in what percentage? WHich is the only functional source for B12 uptake?
2. Describe uptake of B12?
3. What happens after B12 is taken up into cells?
1. Transcobalamin I (TCI) - storage form, turns B12 over very slowly ~75% of B12 is bound to this form; TCII - only functional source for B12 uptake.
- 2. TCII-B12 complex is taken up by receptor-mediated endocytosis (pinocytosis)
- 3. Lysosomes free B12 from complex and it immediately binds to methionine synthase or methylmalonyl CoA mutase
1. Is there a pool of free B12 in cells?
2. What causes deficiency of B12? (4)
3. What can deficiency lead to? What are symptoms of this disease?
- 1. NO
- 2. Loss of parietal cell function (no IF), vegan diet, pancreatic insufficiency, intestinal diseases
- 3. Deficiency can lead to megaloblastic (pernicious) anemia - diarrhea, fatigue, loss of appetite, shortness of breath.
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