Answer- Three plasma transport proteins have been identified. Transcobalamine I and III (differing only in carbohydrate structure) are secreted by white blood cells. Although approximately 90% of plasma vitamin B12 circulates bind to these proteins, only Transcobalamine II is capable of transporting vitamin B12 into cells.
Q.2- Name any two inhibitors of conjugase enzyme, that is required for the removal of extra Glutamic acid residues from the dietary folic acid?
Answer- Conjugase (γ-glutamyl carboxypeptidase) in the intestinal lumen cleaves off extra Glutamic acid residues, and folate is absorbed by the mucosa of the small intestine. Alcohol and Anticonvulsant drugs, such as Dilantin, interfere with mucosal conjugase, hence impair folate absorption.
Q.3- Which form of folic acid is required for the de novo synthesis of purine nucleotide?
Answer- Two different forms of folate are required for different aspects of nucleotide biosynthesis. N10-formyl THF provides the C-2 and C-8 carbons for the de novo synthesis of purine rings, and thus is critical for DNA metabolism. The Methylene form, N5,N10-methylene THF, is required for the production of dTMP from dUMP (pyrimidine nucleotide) .
Q.4- Which form of vitamin B12 is present in plasma ?
Answer- Cobalamin (vitamin B12) exists in a number of different chemical forms. All have a cobalt atom at the center of a corrin ring. In nature, the vitamin is mainly in the 2-deoxyadenosyl (ado) form, which is located in mitochondria. It is the cofactor for the enzyme Methylmalonyl CoA mutase. The other major natural cobalamin is methylcobalamin, the form in human plasma and in cell cytoplasm. It is the cofactor for methionine synthase. There are also minor amounts of hydroxocobalamin to which methyl- and Adenosyl cobalamin are rapidly converted by exposure to light.
Q.5- Why is it said that the requirement of riboflavin increases with the increasing protein load ?
Answer-Riboflavin is required for the catabolism especially the oxidative deamination of amino acids. Thus its requirement increases with the increasing protein load.
Q.6- Why is it said that the maize eaters must supplement with niacin ?
Answer- Maize lacks tryptophan which is a precursor of niacin in the body (60 mg of tryptophan is required for the synthesis of 1 mg of niacin). Maize does contain niacin but it is present in the bound form and is thus not available for biological use. Thus Maize eaters generally manifest pellagra like rashes due to underlying niacin deficiency.
Q.7-What is the basis of using B6-P as a drug for treating infantile convulsions?
Answer- B6-P is required for the decarboxylation of amino acids. Glutamic acid undergoes decarboxylation under the activity of decarboxylase enzyme in the presence of B6-P to form GABA (gamma amino butyric acid) which is an inhibitory neurotransmitter. Thus there is relief of convulsions which is a state of hyper excitation, by B6P through promoting the synthesis of GABA.
Q.8-Which vitamin is required for the absorption of amino acids from the gut?
Answer- Vitamin B6 is required for the active absorption of amino acids from the gut.
Q.9- Which vitamin is required as coenzyme for the activity of lactate dehydrogenase enzyme?
Answer- Lactate dehydrogenase catalyzes the interconversion of pyruvate and lactate. Niacin in the form of NAD+ is required as a coenzyme for the activity of lactate dehydrogenase enzyme.
Q.10-Name the enzyme catalyzing the activation of vitamin B6
Answer- Pyridoxal kinase catalyzes the conversion of B6 to B6-P, the metabolically active form of Vitamin B6.
Q.11- Name the synthetic form of vitamin K.
Answer- Three compounds have the biological activity of vitamin K phylloquinone, the normal dietary source, found in green vegetables; menaquinones, synthesized by intestinal bacteria, with differing lengths of side chain; and menadione and menadiol diacetate, synthetic compounds that can be metabolized to phylloquinone.
Q.12- Achlorhydria can precipitate the deficiency of Vitamin B12, is it true?
Answer- Intrinsic factor (IF) is required for the absorption of vitamin B12 which is secreted by the parietal cells of stomach, In conditions of gastric atrophy due to genetic causes or auto immune destruction, apart from HCl , intrinsic factor is also not synthesized with the resultant impaired absorption of vitamin B12 and pernicious anemia as a consequence.
Q.13- Name the inhibitors of vitamin K that act as anticoagulants.
Answer- Although vitamin K is a fat-soluble vitamin, the body stores very little of it, and its stores are rapidly depleted without regular dietary intake . The oral anticoagulants, such as Dicumarol and warfarin, inhibit coagulation through antagonism of the action of vitamin K.
Q.14-The synthesis of which vitamin is inhibited by Sulphonamides?
Answer- The synthesis of folic acid is inhibited by sulphonamides (sulphanilamides). Because its shape is similar to that of p-aminobenzoic acid, sulfanilamide inhibits the growth of bacteria by interfering with their ability to use p-aminobenzoic acid to synthesize folic acid. Sulfa drugs were the first antimetabolites to be used in the treatment of infectious disease. Because humans don’t make folic acid, sulfanilamide is not toxic to humans in the doses that inhibit bacteria. This ability to inhibit bacteria while sparing humans made them useful in preventing or treating various infections.
Q.15- Which form of vitamin A -11 cis Retinal or 11-cis Retinol is required for vision?
Answer- Vitamin A, in the strictest sense, refers to retinol. However, the oxidized metabolites, retinaldehyde and retinoic acid, are also biologically active compounds. The term retinoids includes all molecules (including synthetic molecules) that are chemically related to retinol. Retinaldehyde (11-cis) is the essential form of vitamin A that is required for normal vision, whereas retinoic acid is necessary for normal morphogenesis, growth, and cell differentiation.
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