002 Vitamin C.pdf

Nutritional Biochemistry Department of Medical Technology, Biochemistry Cluster – 2022-2023 What is Nutrition? Composition and quantity of food intake by living organisms Biochemical utilization of food Human nutrition is divided into three areas: Undernutrition (nutrient defic...

Nutritional Biochemistry Department of Medical Technology, Biochemistry Cluster – 2022-2023 What is Nutrition? Composition and quantity of food intake by living organisms Biochemical utilization of food Human nutrition is divided into three areas: Undernutrition (nutrient deficiency) Overnutrition (excessive nutrient intake) Optimal nutrition (balanced nutrient intake)  Assessment of malnutrition Malnutrition in humans is measured by: Dietary intake studies: identify people with deficient diets Biochemical studies: identify subclinical nutritional deficiencies Clinical symptoms: identify clinical nutritional deficiencies Dietary Reference Intakes (DRIs) Quantitative estimates of nutrient intakes required to prevent deficiencies and maintain optimal health in populations Recommended by: Food and Nutrition Board of the National Research Council, USA Dietary Reference Intakes (DRIs) DRIs have four standards: Estimated Average Requirement (EAR) Recommended Dietary Allowance (RDA) Adequate Intake (AI) Tolerable Upper Intake Level (UL) Estimated Average Requirement (EAR) The amount of nutrient intake estimated to meet the nutritional requirement of half of the healthy individuals (50%) in an age and gender group Recommended Dietary Allowance (RDA) The amount of nutrient intake that is sufficient to meet the nutritional requirement of nearly all (97-98%) healthy individuals in a group RDA is two SD above EAR RDA = EAR + 2 SD Adequate Intake (AI) It is used instead of EAR and RDA if: A nutrient is considered essential, but the experimental data are inadequate for determining EAR and RDA AI covers the nutritional requirement of all individuals in a group with approximation due to insufficient data Tolerable Upper Intake Level (UL) The highest level of daily nutrient intake that has no adverse health effects or toxicity in almost all individuals Water-Soluble Vitamins Vitamin C Department of Medical Technology, Biochemistry Cluster – 2022-2023  Vitamin C known as ascorbic acid or ascorbate Unlike most mammals, humans cannot make their vitamin C. Therefore, we must obtain vitamin C through our diet.  Vitamin C Ascorbic acid functions as a cofactor, enzyme complement, co- substrate, and a powerful anti-oxidant in various reactions and metabolic processes. It also stabilizes vitamin E and folic acid and enhances iron absorption. It neutralizes free radicals and toxins as well as attenuates inflammatory response, including sepsis syndrome  Vitamin C 1 During the oxidation of ascorbic acid, a free radical called ascorbyl radical (ascorbate free radical) is formed but has a short half- life and reacts poorly with oxygen, so it does not form reactive oxygen species. Vitamin C 2 Oxidation of the radical forms of vitamin C. Vitamin C 3 Dehydroascorbic acid can be reduced to ascorbic acid with hydrogens provided by the reduced form of glutathione (GSH).  Vitamin C Vitamin C have a number of vitamers that have vitamin C activity in animals: Ascorbic acid in biological system (low pH) Oxidized form deyhroascorbic acid. Ascorbate in neutral pH Ascorbate and ascorbic acid are both naturally present in the body since the forms interconvert according to pH. vitamer is a chemically similar substance that exhibits similar vitamin activity to a specific vitamin  History of Vitamin C The problems referred to as scurvy and associated with the lack of vitamin C had been quite prevalent for centuries. Some of the most notable stories are those of the British sailors who frequently died from scurvy on sea voyages. It was a physician who eventually found that sucking the juice from a lime was protective, and in the late 1790s and early 1800s British sailors at sea began receiving limes (resulting in the nickname “limey” for the sailors) in an effort to prevent scurvy.  Vitamin C Vitamin C exists as both a D- and L-isomer; however, it is the L- isomer of the vitamin that is biologically active in humans. The inability to synthesize vitamin C results from the lack of gulonolactone oxidase, the last enzyme in the vitamin C synthetic pathway. Synthesis of Ascorbic Acid Vitamin C Content of Selected Foods*  Sources Fruits, vegetables and organ meats (liver, kidney). The best food sources of vitamin C include asparagus, papaya, oranges, cantaloupe, cauliflower, broccoli, green peppers, grapefruit, grapefruit, lemons, and strawberries. Citrus products are most commonly cited as significant sources of the vitamin. Supplements supply vitamin C typically as free ascorbic acid, calcium ascorbate, sodium ascorbate, and ascorbyl palmitate. Sensitivity of Vitamin C Very significant losses occur as vegetables wilt, or when they are cut, as a result of the release of ascorbate oxidase from the plant tissue. Significant losses of the vitamins also occur in cooking, both through leaching into the cooking water and also atmospheric oxidation, which continues when foods are left to stand before serving.  Absorption and Transport Vitamin C does not require digestion prior to being absorbed into intestinal cells.  Absorption and Transport Actively absorbed in the small intestine and requires sodium-dependent vitamin C transporters (SVCT) 1 and 2. SVCT1 is primarily responsible for vitamin C absorption in the intestines, while SVCT2 is involved in transporting vitamin C into various tissues in the body.  Absorption and Transport dehydroascorbic acid is rapidly reduced to ascorbic acid by dehydroascorbic acid reductase in the intestinal cell. SVCT1 is primarily responsible for vitamin C absorption in the intestines, while SVCT2 is involved in transporting vitamin C into various tissues in the body. Vitamin C 3 Dehydroascorbic acid can be reduced to ascorbic acid with hydrogens provided by the reduced form of glutathione (GSH). Absorption and Transport dehydroascorbic acid is rapidly reduced to ascorbic acid Absorption of ascorbate may be by dehydroascorbic acid diminished in the presence of reductase in the intestinal cell. high intracellular glucose, which appears to interfere with ascorbate transporter. Absorption and Transport From intestinal cells, ascorbate may diffuses through anion channels into extracellular fluid and enters plasma by way of capillaries.  Absorption and Transport In cells: Uptake of ascorbate into body cells requires sodium and a carrier and into some cells such as leukocytes, uptake is also energy dependent. Dehydroascorbic acid uptake into cells utilizes GLUT transporters. Tissue concentrations of vitamin C usually exceed plasma concentrations. SVCT1 is expressed in epithelial tissues including the intestine, liver, and kidney and is responsible for most ascorbate transport, whereas SVCT2 is expressed in brain, lung, heart, eye, placenta, neuroendocrine and exocrine tissues, and endothelial tissues.  Absorption and Transport Ascorbate and dehydroascorbate concentrations are much greater in some tissues than others. The highest in adrenal and pituitary glands (~30-50mg/100 g of wet tissue). Intermediate in liver, spleen, heart, kidneys, lungs pancreas and leukocytes. Smaller amounts in muscles and red blood cells. Maximal vitamin C pool is estimated at about 1.500mg.  SUMMARY OF ABSORPTION Absorbed in the intestine by an energy-requiring, sodium- dependent, carrier-mediated transport system. After absorption, ascorbic acid is then transported as a free acid in plasma into the cells, including leukocytes and red blood cells. In the tissues, Vitamin C serves as an electron donor for many enzymes.  Function & Mechanism of Action Vitamin C has very complex functional roles in the body as a cofactor in around eight reactions: Collagen synthesis Drug and steroid metabolism. Carnitine synthesis Maintain the iron and copper Tyrosine synthesis and atoms in the metalloenzymes in catabolism the reduced state. Neurotransmitter synthesis. Collagen The major protein in the extracellular matrix Additional reading: collagen synthesis  Collagen Synthesis STEPS The process begins in specialized cells, such 1 as fibroblasts (skin cells) or osteoblasts (bone cells). Translation of pre-pro-collagen happens This chain then travels to the endoplasmic 2 reticulum (ER) for post-translational modification.  Collagen Synthesis STEPS The process begins in specialized cells, such 1 as fibroblasts (skin cells) or osteoblasts (bone cells). Translation of pre-pro-collagen happens This chain then travels to the endoplasmic 2 reticulum (ER) for post-translational modification.  The signal peptide on the N-terminal is removed, and the molecule now is known as propeptide The lysine and proline residues get additional hydroxyl groups added to them via hydroxylase enzymes which require vitamin C as a cofactor Glycosylation of the selected hydroxyl groups on lysine with galactose and glucose b Three of the hydroxylated and glycosylated pro-a-chains assemble by twisting into a triple helix by zipper-like folding. Now the pro-collagen molecule is ready to move to the Golgi apparatus for final modifications and assembled into secretory vesicles to enter the extracellular space  Collagen Synthesis For the collagen molecule to aggregate into its triple-helix configuration selected proline residues must be hydroxylated forming hydroxyproline. Requires di-oxygenase enzymes, reduced iron (Fe+2), ascorbate.  Collagen Synthesis Vitamin C role: During hydroxylation, the iron cofactor in the enzymes is oxidized. Ascorbate acts as a reducing agent to convert the oxidized Fe3+ back to its reduced state Fe2+ in the enzymes lysyl hydroxylase and prolyl hydroxylase Collagen Synthesis Vitamin C may also influence mRNA levels needed for collagen synthesis. Although these reactions may seen as simple, normal development and maintenance of skin, tendons, cartilage, bone, and dentine depend on an adequate supply of vitamin C. Also, important in wound healing and bleeding prevention from capillaries. Carnitine Synthesis Carnitine is a methylated from nitrogen containing compound made from lysine. Sufficient carnitine is critical in fat metabolism because it is essential to transport long-chain fatty acids from cell cytoplasm into the mitochondrial matrix where β-oxidation occurs.  Carnitine Synthesis Required for two hydroxylation reactions in synthesis carnitine, which functions as the preferred reducing agent, specifically reducing Fe from ferric (Fe3+ ) back to ferrous state (Fe2+ ). Tyrosine Tyrosine is a nonessential amino acid the body makes from another amino acid called phenylalanine. ingested from foods or generated in the body from phenylalanine oxidation, can be degraded if needed to produce energy. It is an essential component to produce several important brain chemicals called neurotransmitters, including epinephrine, norepinephrine, and dopamine. Tyrosine catabolism occurs in liver and kidney. Tyrosine Catabolism  Neurotransmitter Synthesis Vitamin C maintains mineral cofactors for some of the enzymes involved in the synthesis of neurotransmitters in its reduced state. Norepinephrine It is generated from the hydroxylation of the dopamine side chain. This reaction is catalyzed by dopamine monoxygenase (containing 8 Cu atoms)-vitamin C- dependent reaction. Found in nervous tissue and adrenal medulla.  Neurotransmitter Synthesis Serotonin Tryptophan is an amino acid important for the production of serotonin in the body. It is also key to brain function and plays a role in healthy sleep. The first step of serotonin synthesis requires tryptophan mono-oxygenase (hydroxylase), O2, tetrahydrobiopterin, and vitamin C.  Neurotransmitter Synthesis Other Neurotransmitters and Hormones Vitamin C also serves as a reductant, keeping the copper atom in peptidylglycine α-amidating monooxygenase in its reduced state. Many of amidated peptides are active as hormones, such as calcitonin, cholecystokinin (CCK), and gastrin. The enzyme found in the pituitary, adrenal, thyroid glands, and brain.  SUMMARY Vitamin C acts as an electron donor for 8 different enzymes: Three enzymes participate in collagen hydroxylation. Two enzymes are necessary for the synthesis of carnitine. Three remaining enzymes have the following functions in common, but have other functions as well: Dopamine β-hydroxylase participates in biosynthesis of norepinephrine from dopamine. Enzyme adds amide groups to peptide hormones. One modulates tyrosine metabolism.  Microsomal Metabolism A group of enzymes makes up a microsomal metabolizing system, that mostly functions in the liver, to inactivate both endo- and exogenous substances. Endogenous: Include various hormones and steroids Exogenous: xenobiotics drugs, carcinogens, pesticides, pollutants, food additives. The reactions to metabolize these substances usually involve hydroxylation followed by other reactions to produce polar metabolites for excretion.  Antioxidant Activity Vitamin C’s two hydroxyl groups and carbonyl group facilitate its ability to act as a hydrogen or electron donor. The reduction potential of ascorbate is such that it readily donates electrons/hydrogen ions to regenerate other antioxidants, such as vitamin E, glutathione, and uric acid, and to reduce numerous ROS and RNS.  Antioxidant Activity Ascorbic acid interacts with oxidants in the aqueous phase (blood or intracellular) before they initiate damage in the nucleus, and cell lipids. Ascorbic acid appears to be superior to other water-soluble antioxidants. Ascorbate = thiols > bilirubin > uric acid > vitamin E  Pro-Oxidant Activity Vitamin C can reduce transition metals, while itself becoming oxidized to semi-dehydroascorbate: Fe3+ or Cu2- Semidehydroascorbate radical (AH−) + Fe2+ or Cu1+ These reduced metal ions can cause cell damage by generating ROS and free radicals. Fe2+ or Cu1+ + H2O2 Fe3+ or Cu2- + H2O2 + OH Fe2+ or Cu1+ + O2 Fe3+ or Cu2- + O2-  Other Function Vitamin C and colds: Vitamin C is thought to moderate colds by enhancing many immune cells (such as some leukocyte) functions while also destroying histamine, which causes many of a cold’s symptoms.  Other Function Vitamin C and cancer: Some have shown that survival time in cancer time patients may be prolonged, possible protective mechanisms Ability to act as a reducing agent Detoxify carcinogens.  Toxicology Excretion is reduced when intake is low Urinary excretion Body pool < 1.500mg leads to only metabolites in urine Body pool > 1500mg leads to proportionately more ascorbate in urine Vitamin C may be excreted intact or oxidized by L-ascorbate oxidase to oxalic acid, primarily in the liver and some in the kidney. Toxicology Vitamin C and the formation of its metabolites excreted in the urine.  Vitamin C RDA Adult male: 90mg Adult female: 75 mg Pregnancy: 100my Lactating: 120mg Smokers: extra 35 mg UL: adult 2000 mg.  Deficiency Scurvy Scurvy leads to the formation of brown spots on the skin, bleeding gums, small red skin discolorations caused by ruptured small blood vessels, sublingual hemorrhages, easy bruising, impaired wound and fracture healing, joint pain, loose and decaying teeth, and hyperkeratosis of hair follicles, especially on the arms, legs, and buttocks.  Deficiency Scurvy  Toxicity Toxicity is rare. Tolerable upper intake level of 2g vitamin C / day The most common side effect with ingestion of large amounts (2 g) of the vitamin is gastrointestinal problems characterized by abdominal pain and osmotic diarrhea. Other side effects include an increased risk of kidney stones and iron toxicity for those with renal disease and disorders of iron metabolism. THANK YOU! Department of Medical Technology, Biochemistry Cluster – 2022-2023