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Vitamins:The word Vitamin comes from the Greek word “VITAMINE” which means ‘Vital for Life.’
Vitamins are organic compounds, they are essential for the normal processes of metabolism, including growth and maintenance of health. Vitamins are required in small amount, they must be taken in diet because the body either cannot synthesized them or synthesis are insufficient. E.g. of vitamins synthesized by the body are vitamin D synthesized from cholesterol and niacin from tryptophane.
Vitamins are divided in to 2 main types:
Water soluble vitamins:
They include B-complex vitamins and vitamin C. because their water-solubility, these vitamins have no stable storage form and must be provided continuously in diet. Vitamin B12 is an exception in that normal human liver can store it several years. as they are water soluble, so excess concentration are excreted in urine so rarely accumulated in toxic concentration.The water soluble vitamins form coenzymes ( active form of the vitamin) that participate in a variety of biochemical reactions.
Coenzyme: Small organic molecule that associates closely with enzyme functions.
B-complex vitamins:Thiamine (B1):
Structure And Active Form
Thiamine contains a pyrimidine ring and a thiazole ring.
The active form of thiamine is the coenzyme Thiamine Pyrophosphate (TPP) which is formed when the OH group of thiamine reacts with two phosphates (pyrophosphates) donated by ATP.
The reaction is catalysed by enzyme thiamine pyrophosphate transferase.
Activation occurs mainly in liver, and also in the brain.
Dietary thiamine is readily absorbed and phosphorylated to its active form thiamine diphosphate (thiamin pyrophosphate) this needs to the presence of ATP dependent thiamine diphosphotransferase, that present in the brain and liver.
Thiamin pyrophosphate
Sources:Whole grain cereals, legumes, yeast, liver, meat.
Unpolished Rice and Whole wheat bread
RDA: 1 to 1.5mg per day
Destroyed by alkaline and heat
Biochemical function:
Thiamine diphosphate is an important coenzyme in carbohydrate metabolism , being necessary for 2 types of reaction:
1. oxidative decarboxylation reactions (e.g. conversion of pyrovate to acetyl CoA)
2. Transketolation reactions e.g. transketolase catalyzed reactions of the pentose phosphate pathway.
Reactions requiring TPP
DECARBOXYLATION REACTIONS
Pyruvate Dehydrogenase (PDH)
α Ketoglutarate dehydrogenase.
TRANSKETOLATION REACTION
3. Transketolase.
Deficiency of thiamin:
Causes of Deficiency ManifestationInadequate Diet (polished rice): Thiamine is present in the outer layer of rice grains.
Alchoholism: Decreased absorption because of intestinal mucosal changes. Also causes liver cirrhosis resulting in no activation of thiamine.
Pregnancy and lactation: Leads to increase demand.
Presence of enzyme thiaminase ( in raw fish) inactivates thiamine by breaking the thiazole ring.
Increased Carbohydrate intake ( more requirement of thiamine).
Increased metabolic activity due to Hyperthyroidsm and fever
In the thiamine deficient human, the thiamine pyrophosphate dependent reactions are prevented or severely limited ,leading to accumulation of the substrate of the reactions e.g. pyrovat.
Clinical Significances of Thiamine Deficiency
Deficiency of thiamine causes beriberi in which anorexia, nausea , as well as mental depression, peripheral neuropathy and fatigue. This form called dry beriberi .wet beriberi there is peripheral oedema, sometimes associated with cardiac failure.
Beriberi, may be aggravated by a diet that is carbohydrate rich and thiamin deficient, possibly because this leads to an increased rate of glycolysis and therefore pyrovate production.
2. An additional thiamine deficiency related disease is known as Wernicke-Korsakoff syndrome
: occur due to thiamine deficiency, it found in chronic alcoholics consuming little food, which characterized by apathy, loss of memory, and nystagmus.
Riboflavin .
vitamin B2
Riboflavin structure
It is relatively heat stable but sensitive to irreversible decomposition when exposed to visible light.
Biochemical function :
Riboflavin is the precursor for the coenzymes, flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD). The enzymes that require FMN or FAD as cofactors are termed flavoproteins. Several flavoproteins also contain metal ions and are termed metalloflavoproteins. Both classes of enzymes are involved in a wide range of oxidation-reduction reactions. During the course of the enzymatic reactions involving the flavoproteins the reduced forms of FMN and FAD are formed, FMNH2 and FADH2, respectively.
Structure of FAD
The normal daily requirement for riboflavin is 1.2 - 1.7 mg/day for normal adult.The coenzymes FMN(FMNH2) and FAD(FADH2) participate in many oxidation reduction reactions and in the Electron Transport Chain.
Example:
FAD FADH2
Succinate Fumarate
Succinate Dehydrogenase“This reaction takes place in the TCA cycle”
Sources:
Milk and milk products, meat, eggs, liver are rich sources.
Cereals, fruits, vegetables and fish are moderate sources.
Causes of Deficiency
Phototherapy: Riboflavin being light sensitive gets destroyed. This characteristic can lead to riboflavin deficiencies in newborns treated for hyperbilirubinemia by phototherapy.
Chronic Alcoholics: Because of intestinal mucosal and liver damage activation to the coenzyme forms (FMN and FAD) does not occur.
Pregnancy and Lactation: There is increased demand of Riboflavin
Chlorpromazine Therapy: Antipsychotic drug that competitively inhibits activation.
Because of wide distribution (many sources) dietary deficiency is not common”.
Deficiency Manifestation
Mostly seen with other vitamin Deficiencies.
Glossitis: Smooth and purplish tongue.
Circumcorneal Vascularization and conjunctivitis.
Dermatitis: Inflammation of the facial skin in particular
Cheilosis: Fissures at the corner of the mouth
Summary:
1. vitamins are essential organic compounds
2. vitamins should be supplied in diet
3 .two major types of vitamins:
a.water sluble (B-complex and vit. C).
b. fat solube. (A,K,E,D)
4. active form of thiamin is thiamin pyrophosphate.
5. active form of riboflavin are FMN and FAD.