Fat Soluble Vitamins MCQs

Vitamin D is not strictly a vitamin since it can be synthesised in the skin, and in most conditions, this is the major source of the vitamin. Only when sunlight exposure is inadequate, a dietary source is required. Its main function is in the regulation of calcium absorption and homeostasis most of its actions are mediated by way of nuclear receptors that regulate gene expression. It also has a role in regulating cell proliferation and differentiation. There is evidence that intakes considerably higher than are required to maintain calcium homeostasis reduce the risk of insulin resistance, obesity, and the metabolic syndrome, as well as various cancers. Deficiency, leading to rickets in children and osteomalacia in adults, is a problem in northern latitudes, where sunlight exposure is inadequate Reference: Harper's Illustrated Biochemistry, 32nd Edition, Page Number- 1297.

A number of other proteins undergo the same vitamin K-dependent carboxylation of glutamate to y-carboxyglutamate which include: Osteocalcin Protein C and S Matrix Gla protein in bone Nephrocalcin in kidney and Growth arrest specific gene Gas6, which is involved in both the regulation of differentiation and development in the nervous system, and control of apoptosis in other tissues. All of these y-carboxyglutamate containing proteins bind calcium, which causes a conformational change so that they interact with membrane phospholipids. Reference: Harper's illustrated biochemistry 32 nd edition; page 390, Lippincott illustrated reviews 8 th edition biochemistry page 1305

The lack of Vit. E causes the membrane of the RBC to be abnormally fragile and as a result of lipid peroxidation, there is membrane lysis causing haemolytic anemia Reference: Harper's illustrated biochemistry 32 nd edition, page 1302

Fat soluble vitamins are absorbed in Chylomicrons Most clinical manifestations of abetalipoproteinemia result from defects in the absorption and transport of fat-soluble vitamins. Vitamin E and retinyl esters are normally transported from enterocytes to the liver by chylomicrons. Vitamin E is dependent on VLDL for transport out of the liver and into the circulation. As apo B containing lipoproteins are not formed these patients, there is, marked deficiency of Vitamin E, mild to moderate deficiency of Vitamin A and K. Bleeding manifestation due to defective absorption of fat and hence fat-soluble vitamins. Reference: Harper's illustrated biochemistry 32 nd edition, page 275

In phase 1 reactions, xenobiotics are converted to more polar, hydroxylated derivatives. In phase 2 reactions, these derivatives are conjugated with molecules such as glucuronic acid (Option A), sulfate, glutathione (Option B), Glycine (Option D), Acetyl Co A, Methyl group etc. This renders them even more water-soluble, and they are eventually excreted in the urine or bile. But glucose is directly not used as conjugating agent. So, Option C is correct. Reference: Harper's illustrated biochemistry 32nd edition CH 47 PN 558

The main reaction involved in phase 1 detoxification metabolism is hydroxylation, catalyzed by a family of enzymes known as monooxygenases or cytochromes P450. Option A: At least half of the common drugs that we ingest are metabolized by isoforms of cytochrome P450. Option B: They also act on steroid hormones, carcinogens, and pollutants. Option C: Cytochromes P450 are important in the metabolism of a number of physiological compounds-for example, the synthesis of steroid hormones and the conversion of vitamin D to its active metabolite, calcitriol. Option D: Vitamin E metabolism do not involve role of cytochrome P450. A conversion of vitamin D to its active metabolite, calcitriol required cytochrome P450 enzyme. Reference: Harper's illustrated biochemistry 32nd edition CH 47 PN 557

Excess Vit. D is the most toxic vitamin when taken in excess as it causes abnormal calcification of soft tissues. Excess Vit. Dcan lead to contraction of blood vessels, high blood pressure, and calcinosis-the calcification of soft tissues. In at least a few cases, hypercalcemia in response to low intakes of vitamin D is due to genetic defects of calcidiol 24-hydroxylase, the enzyme that leads to inactivation of the vitamin. Although excess dietary vitamin D is toxic, excessive exposure to sunlight does not lead to vitamin D poisoning, because there is a limited capacity to form the precursor, 7-dehydrocholesterol, and prolonged exposure of previtamin D to sunlight leads to formation of inactive compounds. Reference: Harper's illustrated biochemistry 32nd edition, page 1300

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, Menaquinones are absorbed to some extent, but it is not clear to what extent they are biologically active Menadione and menadiol diacetate, synthetic compounds that can be metabolized to phylloquinone. Quinolone is an antibiotic and has no relation to Vit. K Reference: Harper's illustrated biochemistry 32 nd edition, page 1302