CHAPTER 44: VITAMIN A

OBJECTIVES

  1. Discuss the cause-effect relationship between steatorrhea and a fat-soluble vitamin deficiency.

Steatorrhea is the presence of excess fat in feces. It can lead to malabsorption of fat-soluble vitamins (A, D, E, and K) and cause deficiencies of these vitamins.

  1. Show why retinoic acid can support growth and differentiation of epithelial tissues, and why it cannot replace retinol or retinal in their support of the visual system.

Retinoic acid (RA) is a derivative of retinol (vitamin A) that plays an important role in the control of epithelial cell growth and cellular differentiation. RA can support growth and differentiation of epithelial tissues because it binds to nuclear receptors that regulate gene expression. However, RA cannot replace retinol or retinal in their support of the visual system because RA is not involved in the visual cycle.

(Retinoic acid (RA) is not involved in the visual cycle. The visual cycle is a process in the retina that replenishes the molecule retinal for its use in vision. Retinal is the chromophore of most visual opsins, meaning it captures the photons to begin the phototransduction cascade. RA is a metabolite of vitamin A and is involved in many biological processes such as cell growth and differentiation.)

  1. Identify multiple actions of the different vitamin A oxidation states, and discuss relationships between zinc and vitamin A deficiency.

Vitamin A has multiple oxidation states, including retinol, retinal, and retinoic acid. Retinol is involved in vision and the maintenance of epithelial tissues. Retinal is involved in vision. Retinoic acid is involved in the control of epithelial cell growth and cellular differentiation. Zinc deficiency can lead to vitamin A deficiency because zinc is required for the synthesis of retinol-binding protein (RBP), which transports retinol in the blood.

  1. Explain why dietary carotenoids are an unsatisfactory source of vitamin A to carnivores, and why hypothyroidism might induce a state of b-carotenemia in herbivores and omnivores.

Dietary carotenoids are an unsatisfactory source of vitamin A to carnivores because they cannot convert carotenoids to vitamin A efficiently. Hypothyroidism might induce a state of β-carotenemia in herbivores and omnivores because thyroid hormone is required for the conversion of β-carotene to vitamin A.

  1. Indicate how vitamin A is absorbed from the intestine, carried in plasma, taken-up by target cells and bound intracellularly. Also discuss the cellular mechanisms of vitamin A action.

Vitamin A is absorbed from the intestine and carried in plasma bound to retinol-binding protein (RBP). The RBP-retinol complex is taken up by target cells via receptor-mediated endocytosis. Intracellularly, retinol is bound to cellular retinol-binding protein (CRBP) and is converted to retinal by retinol dehydrogenase. Retinal is then converted to retinoic acid by retinaldehyde dehydrogenase. Retinoic acid binds to nuclear receptors that regulate gene expression.

  1. Provide reasoning for each of the signs and symptoms of vitamin A toxicity, as well as those for vitamin A deficiency.

Vitamin A toxicity can cause symptoms such as headache, rash, nausea, vomiting, and abdominal pain. Other symptoms may include vision changes, swelling of the bones, bone pain, poor appetite, dizziness, increased intracranial pressure, drowsiness, irritability, skin peeling, hair loss, itchy skin, dry eyes, cracked lips, high levels of fat in the blood, hepatomegaly (enlarged liver), splenomegaly (enlarged spleen), and pseudotumor cerebri.

Night blindness is one of the most common symptoms of vitamin A deficiency. It is caused by the lack of rhodopsin, a pigment in the retina that helps you see in low light conditions.

  1. Explain the participation of vitamin A in the cascade of reactions leading to amplification of light signals in retinal photoreceptors.

Vitamin A is involved in the amplification of light signals in retinal photoreceptors. In the dark, rhodopsin (a protein found in rod photoreceptors) is bound to 11-cis-retinal. When light enters the eye, 11-cis-retinal is isomerized to all-trans-retinal. This triggers a cascade of reactions that leads to amplification of the light signal. Cellular mechanisms of vitamin A action include regulation of gene expression by retinoic acid via nuclear receptors.

  1. Indicate how vitamin A is thought to be involved in bone resorption (see Chapter 45).

Vitamin A can stimulate the formation of bone-resorbing osteoclasts and inhibit their formation. Vitamin A is thought to be involved in bone resorption because retinoic acid (which the body makes from vitamin A) stimulates osteoclasts.

  1. Understand how retinoic acid toxicity affects the pregnant animal.

Retinoic acid (RA) is a metabolite of vitamin A and is involved in many biological processes such as cell growth and differentiation. RA toxicity during pregnancy can cause birth defects because it can interfere with the normal development of the embryo. RA is teratogenic, meaning it can cause malformations in the developing embryo

  1. Explain common features of vitamin A, steroid and thyroid hormone action.

Vitamin A, steroid hormones, and thyroid hormones are all lipid-soluble molecules that can readily diffuse through the cell membranes into any cell in the body. Cellular mechanisms of vitamin A action include regulation of gene expression by retinoic acid via nuclear receptors.