Topic: Vitamins and minerals.
Question: Describe the biochemical role of vitamin A in human body. Add a note on wald’s visual cycle.
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Vitamin A (retinol) plays an important role in many biochemical processes in the human body:
1. Vision: Vitamin A is essential for vision. It is converted to retinaldehyde, then to retinoic acid which binds to retinoic acid receptors in the retina. This activates genes involved in phototransduction – the process by which light signals are converted to nerve signals in the retina. Disruption of the visual cycle leads to night blindness and other visual defects.
2. Gene expression: Retinoic acid, derived from vitamin A, binds to retinoic acid receptors in the nucleus. This regulates the expression of many genes involved in cell growth, differentiation and immunity. Vitamin A helps maintain epithelial tissue, sperm production, immune function and embryonic development through effects on gene expression.
3. Antioxidation: Vitamin A acts as an antioxidant, helping protect cells against damage from free radicals. Carotenoids like beta-carotene, which can be converted to retinol, also have antioxidant properties.
4. Growth and development: Vitamin A is essential for growth and development in children, especially for bone development, teeth development, and surface linings of the respiratory, urinary and intestinal tracts. Deficiency leads to growth retardation, infertility and night blindness.
5. Immune function: Vitamin A is important for the healthy functioning of the immune system. It is required for the development and maintenance of mucosal barriers and normal T-cell mediated immune responses. Both deficiency and excess can negatively impact immunity.
The visual cycle refers to the conversion of vitamin A to retinaldehyde (retinal) and its regeneration. In the retina, light exposure isomerizes retinal to metaretinal, activating photoreceptor cells. Metaretinal then reforms retinal with the enzyme retinal pigment epithelium-specific 65 kDa protein (RPE65). This “recycling” of retinal allows for continual phototransduction and vision in the presence of light.Mutations in the genes for RPE65 and LRAT (lecithin retinol acyltransferase) can disrupt the visual cycle, impairing conversion of retinol to retinal and causing congenital night blindness.
Experimental treatments involving injection of functional RPE65 genes have successfully restored vision in some patients with these genetic conditions.Vitamin A and its derivatives play diverse and vital roles in metabolism, growth, vision, immunity and gene regulation. Maintaining adequate levels is important for health, while excess intake can also be problematic. Understanding these biochemical processes helps scientists determine optimal intake levels, treat deficiency or toxicity, and develop new therapies for conditions like congenital night blindness. The complexity of vitamin A’s actions reflects its fundamental role in sustaining life and promoting wellness.
