(7) B6 and biotin

B6- function

main coenzyme form- PLP, amino acid metabolism, neurotransmitters, fatty acid oxidation, glycogen breakdown, tryptophan metabolism

transamination reactions involve

the transfer of an amino group from one amino acid to an alpha ketoacid, important for the synthesis of nonessential amino acids and for the use of amino acid carbon skeletons for energy and glucose production

dehydration 

PLP also participates in reactions in which an amino group is removed from a compound such as an amino acid and released as ammonia or ammonium ion

decarboxylation reactions

involve the removal of the carboxyl group from an amino acid, involved in the production of neurotransmitters for nervous system function

transulfhydration

PLP is required for the activity of two enzymes, cystathionine b-synthase and cystathionine lyase, catalyzing reactions in the transulfhydration pathway

the two PLP-dependent enzymatic reactions facilitate

homocysteine catabolism and regulation, provide for the synthesis of cysteine, a nonessential amino acid, which is required for the production of proteins, the antioxidant glutathione, and taurine

transelenation

selenomethionine may be converted through the transelenation pathway to selenocysteine

selenocysteine b-lyase

PLP dependent, generates selenide from selenocysteine, selenide is converted into selenophosphate, an important intermediate in the synthesis of the body’s selenium-dependent enzymes, which serve in antioxidant roles

glycogen breakdown

catabolized by glycogen phosphorylase, vitamin B6 is required for glycogen phosphorylase activity, bound to glycogen phosphorylase

tryptophan metabolism

coenzyme for kynureninase- involved in niacin synthesis from tryptophan

B6- RDA

9-13: 1mg, 14-18: 1.3mg

B6- sources

pyridoxine (most stable) found in plant foods, pyridoxal and pyridoxamine are found primarily in animal products

B6- bioavailability

influenced by the food matrix and the extent/type of processing to which the foods are subjected, fairly stable with cooking

B6 deficiency- signs and symptoms 

a seborrheic dermatitis rash on the face, neck, shoulders, and buttocks areas, cheilosis, glossitis, and angular stomatitis, central and peripheral nervous systems

B6 deficiency also impairs

niacin synthesis from tryptophan and inhibits the metabolism of homocysteine, which may result in hyperhomocysteinemia

people at risk for B6 deficiency

older adults, alcoholics, systemic inflammation, malabsorptive conditions

B6 toxicity 

TUL= 100mg/day, can cause sensory and peripheral neuropathy along with problems with movement 

B6 toxicity- signs and symptoms 

unsteady gait, paresthesia in the extremities, impaired tendon reflexes, ataxia, degeneration of sensory fibers in peripheral nerves

biotin- coenzyme for carboxylase

propionyl-CoA carboxylase (a mitochondrial enzyme) is important for the catabolism of four amino acids, odd chain fatty acids, and cholesterol, each generates propionyl-CoA

biotin- pyruvate carboxylase

critical mitochondrial enzyme in gluconeogenesis (in liver) during times of fasting and starvation, catalyzes the carboxylation of pyruvate to form oxaloacetate, surplus of ATP: oxaloacetate is then used for gluconeogenesis, deficient in ATP: oxaloacetate enters the TCA cycle

biotin: methylcrotonyl-CoA carboxylase

breakdown of leucine leads to b-methylcrotonyl-CoA formation, which is then carboxylated by biotin-dependent b-methylcrotonyl-CoA carboxylase to form b-methylglutaconyl-CoA which is catabolized to generate acetoacetate and acetyl-CoA

biotin- Acetyl-CoA carboxylase 1

the regulatory and rate-limiting enzyme acetyl-CoA carboxylase 1, found in the cytosol, is responsible for malonyl-CoA formation from acetyl-CoA

biotin- non coenzyme roles

histone modification, gene expression, cell signaling

biotin- RDA

adults and pregnancy: 30 μg/day, lactation: 35 μg/day

biotin- sources

beef liver, eggs, salmon, pork chop, sunflower seeds, and sweet potatoes

biotin- deficiency 

rare, ingestion on raw egg whites, GI disorders, excessive alcohol intake, symptoms- depression, hallucinations, dermatitis found around the eyes, nose, mouth, alopecia, brittle nails, muscle pain

biotin- toxicity 

no TUL established, interfere with diagnostic tests, specifically causing elevations in thyroid hormone concentrations and causing reductions in serum troponin concentrations