Lecture 15 - micronutrients-3

Lecture 15 - micronutrients-3 - Micronutrients 3...

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Micronutrients 3: Micronutrients that act as enzyme cofactors Niacin, thiamin, riboflavin, B6, folate, B12, biotin, pantothenic acid
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NIACIN
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Niacin (Vit B3) Niacin (Vitamin B3) Discovered through the condition pellagra in humans and a similar condition, called black tongue, in dogs The “anti-black tongue” factor High incidence in areas where corn is the main dietary staple (because niacin is bound to complex carbohydrates and poorly absorbed) Generic term for: Nicotinic acid and Nicotinamide Dietary sources? Most fish, meats, breads and cereals, coffee and tea In coffee…trigonelline is converted to niacin by heat (coffee bean roasting) In animal foods, niacin occurs predominantly as: Nicotinamide Nicotinamide adenine dinucleotide (NAD) Nicotinamide adenine dinucleotide phosphate (NADP) In plant foods, niacin predominantly as nicotinic acid Niacin can also be produced in the liver from the AA tryptophan (but only about 1/60 th of Trp converted into niacin)
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Digestion & Absorption Digestion of NAD and NADP required for absorption of niacin Hydrolyzed by glycohydrolase to release free nicotinamide Nicotinic acid and nicotinamide absorbed a bit in stomach, but mostly in small intestine by facilitated diffusion In plasma, niacin found primarily as nicotinamide 1/3 of the nicotinamide bound to plasma proteins Both forms can diffuse across cell membranes in most tissues except kidney and RBC (carrier-mediated) Nicotinamide primary precursor for NAD (all tissues) Nicotinic acid used for NAD in only the liver Once NAD or NADP produced, this essentially traps niacin within the cell. Typically found as: NAD+ (oxidized form) / PRIMARY FUNCTION? Transfer electrons to ETC NADPH (reduced form) / PRIMARY FUNCTION? Reducing agent in biochemical pathways
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NAD Production From either NA or Trp Two steps: 1. Convert the acid to an amide 2. Build into a dinucleotide structure NAD+ reduced to NADH in: Glycolysis, Kreb’s cycle, B- oxidation CATABOLIC NADP+ reduced to NADPH in hexose monophosphate shunt and used for: Fatty acid synthesis, DNA synthesis, glutathione regeneration, etc. ANABOLIC Accept 2 electrons Over 200 enzymes require NAD and NADP as coenzymes NA (nicotinic acid) 1 2 NAD+ (oxidized) >> NADH NADPH (reduced) >> NADP+
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Niacin Requirements / Deficiency Corn/Maize Contains significant amounts of Vit B3, but it’s not readily absorbed Also deficient in Trp Use of lime (from limestone, not the fruit) can help release Vit B3 from corn Used by Native Americans in food practices Deficiency leads to pellagra (4 Ds): dermatitis, dementia, diarrhea, death – reversible RDA recommendations include niacin produced from Trp Therefore…Niacin Equivalent (NE) NE = mg preformed niacin + mg Trp/60 14/16mg per day NE women/men
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RIBOFLAVIN
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Riboflavin (Vit B2) Riboflavin (Vitamin B2) Riboflavin stems form “ribo” (ribose-like side chain) + “flavus” (yellow colour)
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