Flashcards in Nutrition Deck (302)
Give a general description of Vitamin A
-collectively called retinoids
-Retinol: transport and storage form of vitamin A
-11-cis retinal is required for vision
-Retinol and retinal can be easily interconverted.
-Retinoic acid: epithelial growth and differentiation. It has steroid hormone like effect.
Describes the steps of the absorption and transport of Vitamin A
Diet contain Retinol esters -> hydrolysis by intestinal mucosa releasing Retinol and free fatty acids -> re-esterfication and secretion in chylomicrons which are taken up by the liver where it is stored -> plasma Retinol binding protein in the liver transports to extra hepatic tissues -> tissues contain cellular Retinol binding protein that carries Retinol into the cell
List the sources of Vitamin A
-Liver, kidney, butter and cream products; egg yolks
-Yellow vegetables and fruits ( dietary pro vitamin form: B-carotene)
What are the functions of Vitamin A?
-Vision: 11 cis retinal is a component of rhodopsin (visual pigment). Visual cycle req isomerization b/w cis and trans-retinal
-Maintenance of specialized epithelia, esp mucus secreting cells (Retinoic acid)
-Growth (Retinoic acid)
Describe the mechanism of action of Retinoic acid in epithelia cells.
1) Retinol enters target cell and is oxidized to Retinoic Acid in cytosol
2) Retinoic Acid moves from cytosol into nucleus with help of Retinoid Binding Proteins
3) Retinoic Acid binds to nuclear receptors forming an activated receptor complex
4) Retinoic acid-receptor complex binds to chromatin activating the transcription of specific genes (keratin)
Describes the steps in the visual cycle pathway
-Retinol (transported in blood in the form of all-trans Retinol) enters retinal pigment epithelium.
-Esterfied to All-trans retinyl ester
-becomes 11-cis Retinol
- oxidized to 11-cis Retinal
- 11-cis retinal enters rod cell where it combines with opsin to form rhodopsin (visual pigment).
- absorption of light photon catalyzes isomerization of 11-cis retinal to all-trans Retinal triggering eventual signal for vision.
-Opsin becomes available again and All-trans retinal eventually becomes all-trans Retinol.
- all trans Retinol goes back to retinal pigment epithelium to renter cycle.
What causes Vitamin A deficiency?
Dietary deficiency most common - fat free diet
What are some signs and symptoms of Vitamin A deficiency?
-increased risk of pulmonary infections
- immune deficiency - weakened innate immunity
What is Night blindness and what is the cause?
Earliest of the symptoms of Vitamin A deficiency.
-inability to see Dim light
What is Xerophthalmia and what is the cause?
Vitamin A deficiency that produces dryness of the conjunctiva and cornea.
What treatment is Retinoic acid given for?
Acne treatment, especially topical form
What is all-trans Retinoic acid used in treatment of?
Acute promyelocytic leukemia
What can hypervitaminosis of Vitamin A cause?
-Raised intracranial pressure leading to headaches - may mimic brain rumors
-Dry and pruritic skin
-can cause spontaneous abortions and congenital malformations in the fetus during pregnancy.
What is the name for vitamin D found in plants?
What is the general name for vitamin D found in animals?
What is the name of the precursor for Cholecalciferol synthesis in skin?
(intermediate in cholesterol synthesis)
(Cholecalciferol is Vitamin D3)
What are other names for the active form of Vitamin D?
•1,25 dihydroxy cholecalciferol
In Vitamin D synthesis, what is the first step?
7-dehydrocholesterol in the skin is activated by sunlight to form Cholecalciferol.
Cholecalciferol is taken in through diet.
Regarding the steps of Vitamin D synthesis, what happens to Cholecalciferol?
Goes to liver (from skin or diet) and, through enzyme 25-Hydroxylase, becomes 25-hydroxycholecalciferol. Next goes to kidney.
Regarding the steps of Vitamin D synthesis, what happens in the kidney?
25-hydroxycholecalciferol (from liver) is converted into 1,25-dihydroxycholecalciferol/Calcitriol (active form of vitamin D) by the enzyme: 1-Hydroxylase
Which is the regulated step in Vitamin D synthesis and how is it regulated?
25-hydroxycholecalciferol to 1,25-dihydroxycholecalciferol step in kidney.
1-Hydroxylase is activated by:
1) Parathyroid Hormone
2) low calcium levels
1-Hydroxylase is inhibited by:
What occurs once 1,25-dihydroxycholecalciferol (Calcitriol) is made?
(Mechanism of action)
It binds to intercellular receptor proteins
What happens after Calcitrol binds to intracellular receptor proteins of the target cell?
Calcitrol-receptor complex interacts with DNA in nucleus and can either stimulate or repress gene expression.
What effect does Calcitriol have on the intestine?
It stimulates intestinal absorption of calcium and phosphate by increasing synthesis of a specific calcium binding protein.
What effect does Calcitriol have on bone?
It stimulates the mobilization of calcium and phosphate from the bone in the presence of parathyroid hormone.
What effect does Calcitriol have on the kidneys?
It inhibits calcium excretion by stimulating parathyroid dependent calcium reabsorption.
What are the causes of Vitamin D deficiency?
•nutritional deficiency - decreased intake / fat malabsorption
•inadequate exposure to sunlight (common during winter)
•chronic renal disease, chronic liver disease results in decreased hydroxylation of vitamin D
What is Rickets?
Vitamin D deficiency in children.
How does Rickets effect calcium absorption in the diet?
Calcium absorption is decreased, causing:
- increased parathyroid hormone release which leads to
-demineralization of bone.