CHAPTER 40: THIAMIN(B1) AND RIBOFLAVIN(B2)

OBJECTIVES

1. Recognize the two general types of reactions that utilize the activated coenzyme form of thiamin.

The two general types of reactions that utilize the activated coenzyme form of thiamin are decarboxylation and transketolation.

2. Explain why erythrocytes are used to assess thiamin deficiency.

Erythrocytes are used to assess thiamin deficiency because the concentration of total thiamin (free thiamin plus its phosphate esters in whole blood) is 60–120 μg/L, with 90% of the vitamin in erythrocytes and leukocytes. The erythrocytes contain ~80% of the total thiamin in whole blood.

3. Understand why cats fed excessive amounts of raw fish may develop thiamin deficiency.

Cats fed excessive amounts of raw fish may develop thiamin deficiency because raw fish contains thiaminase, which often breaks down thiamine in cats. A deficiency in thiamine can lead to neurological problems and convulsions

4. Associate various animal species with either beriberi, polioencephalomalacia or Chestak's paralysis, and discuss the symptoms and causes of these conditions.

Beriberi is a thiamin deficiency disorder that affects humans. Polioencephalomalacia (PEM) is the most common thiamine deficiency disorder in young ruminant and nonruminant animals. Symptoms of PEM include a profuse, but transient, diarrhea, listlessness, circling movements, stargazing or opisthotonus (head drawn back over neck), and muscle tremors. Chestak’s paralysis is a neurological disease of birds that is caused by thiamine deficiency. The progression of this syndrome includes the inability to keep wings folded, loss of walking ability and flight, seizures, total paralysis, and death.

5. Show how FMN and FAD can be formed from riboflavin and explain what the metalloflavoproteins are.

Flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) can be formed from riboflavin. Riboflavin is converted to FMN by the enzyme riboflavin kinase with ATP as a cofactor. FMN is then converted to FAD by the enzyme FAD synthase with ATP as a cofactor. Metalloflavoproteins are proteins that contain flavin cofactors and metal ions. They are involved in many biological processes such as electron transfer, oxygen transport, and DNA repair.

6. Show how the flavins (FAD and FMN) participate in oxidative phosphorylation (see Chapter 36).

FMN and FAD participate in oxidative phosphorylation by acting as cofactors for enzymes involved in the electron transport chain. The electron transport chain is a series of protein complexes that transfer electrons from electron donors to electron acceptors via redox reactions. The energy released from these redox reactions is used to pump protons across the inner mitochondrial membrane, creating a proton gradient that drives ATP synthesis by ATP synthase.

7. Identify and describe eight different biochemical reactions that would be impaired in riboflavin deficiency and predict the consequences.

Eight different biochemical reactions that would be impaired in riboflavin deficiency include the metabolism of carbohydrates, lipids, and amino acids; the synthesis of niacin from tryptophan; the conversion of vitamin B6 to its active form; the conversion of folate to its active form; the metabolism of iron; and the detoxification of xenobiotics. The consequences of riboflavin deficiency include anemia, dermatitis, glossitis, and neurological symptoms.

8. Identify important dietary sources of riboflavin and explain why it appears in the enterohepatic circulation (EHC).

Important dietary sources of riboflavin include milk and dairy products, meat, eggs, and green leafy vegetables. Riboflavin appears in the enterohepatic circulation because it is excreted in bile and reabsorbed in the small intestine.

9. Explain why erythrocytic glutathione reductase activity can be used to assess a riboflavin deficiency (see Chapter 30).

Erythrocytic glutathione reductase activity can be used to assess a riboflavin deficiency because it is a functional indicator of riboflavin status.