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Biochemistry

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Amino Acids, Proteins and Porphyrins

Question
30 out of 36
 

Thiamine deficiency decreases energy generation because of? (AIIMS May 2008)



A Decrease activity of alpha ketoglutarate and pyruvate dehydrogenase

B It is cofactor in oxidative reduction

C Inhibition Transketolase activity

D It is required for transamination

Ans. A Decrease activity of alpha ketoglutarate and pyruvate dehydrogenase

THIAMINE (VITAMIN B1)

a. Thiamine was the first B vitamin to be identified and is therefore also referred to as vitamin B1.

b. Thiamine functions in the decarboxylation of α-ketoacids, such as pyruvate α-ketoglutarate, and branched-chain amino acids and thus is a source of energy generation.

c. In addition, thiamine pyrophosphate acts as a coenzyme for a transketolase reaction that mediates the conversion of hexose and pentose phosphates.

d. It has also been postulated that thiamine plays a role in peripheral nerve conduction, although the exact chemical reactions underlying this function are unknown.

FOOD SOURCES

a. The median intake of thiamine in the United States from food alone is 2 mg/d.

b. Primary food sources for thiamine include yeast, pork, legumes, beef, whole grains, and nuts. Milled and polished rice contain little thiamine, if any.

c. Thiamine deficiency is therefore more common in cultures that rely heavily on a rice-based diet. Tea, coffee (caffeinated and decaffeinated), raw fish, and shellfish contain thiamineases, which can destroy the vitamin.

d. Thus, drinking large amounts of tea or coffee can theoretically lower thiamine body stores.

DEFICIENCY

a. Most dietary deficiency of thiamine worldwide is the result of poor dietary intake. In western countries, the primary causes of thiamine deficiency are alcoholism and chronic illness, such as cancer.

b. Alcohol is known to interfere directly with the absorption of thiamine and with the synthesis of thiamine pyrophosphate. Thiamine should always be replenished when refeeding a patient with alcoholism, as carbohydrate repletion without adequate thiamine can precipitate acute thiamine deficiency.

c. Thiamine deficiency in its early stage induces anorexia and nonspecific symptoms (e.g., irritability).

d. Prolonged thiamine deficiency causes beriberi, which is classically categorized as wet or dry, although there is considerable overlap.

e. In either form of beriberi, patients may complain of pain and parathesia. Wet beriberi presents primarily with cardiovascular symptoms, due to impaired myocardial energy metabolism and dysautonomia, and can occur after 3 months of a thiaminedeficient diet.

f. Patients present with an enlarged heart, tachycardia, high-output congestive heart failure, peripheral edema, and peripheral neuritis. Patients with dry beriberi present with a symmetric peripheral neuropathy of the motor and sensory systems with diminished reflexes.

g. The neuropathy affects the legs most markedly, and patients have difficulty rising from a squatting position. Alcoholic patients with chronic thiamine deficiency may also have central nervous system manifestations known as Wernicke’s encephalopathy, consisting of horizontal nystagmus, ophthalmoplegia (due to weakness of one or more extraocular muscles), cerebellar ataxia, and mental impairment. When there is an additional loss of memory and a confabulatory psychosis, the syndrome is known as Wernicke-Korsakoff syndrome.

h. The laboratory diagnosis of thiamine deficiency is usually made by a functional enzymatic assay of transketolase activity measured before and after the addition of thiamine pyrophosphate. A 25% stimulation by the addition of thiamine pyrophosphate (an activity coefficient of 1.25)is taken as abnormal. Thiamine or the phosphorylated esters of thiamine in serum or blood can also be measured by high-performance liquid chromatography (HPLC)to detect deficiency.

TREATMENT

In acute thiamine deficiency with either cardiovascular or neurologic signs, 100 mg/d of thiamine should be given parenterally for 7 days, followed by 10 mg/d orally until there is complete recovery. Cardiovascular improvement occurs in 12 h, and ophthalmoplegic improvement occurs within 24 h. Other manifestations gradually clear, although psychosis in the Wernicke-Korsakoff syndrome may be permanent or persist for several months.

TOXICITY

Although anaphylaxis has been reported after high doses of thiamine, no adverse effects have been recorded from either food or supplements at high doses. Thiamine supplements may be bought over the counter in doses of up to 50 mg/d.

Amino Acids, Proteins and Porphyrins Flashcard List

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