Clinical cases and Situational tasks

92. What is the active form of thiamine and how it is formed?

Aswer: Thiamine is mainly the transport form of the vitamin, while the active forms are phosphorylated thiamine derivatives. There are five known natural thiamine phosphate derivatives: thiamine monophosphate (ThMP), thiamine diphosphate (ThDP), also sometimes called thiamine pyrophosphate (TPP), thiamine triphosphate (ThTP), and the recently discovered adenosine thiamine triphosphate (AThTP), and adenosine thiamine diphosphate (AThDP).

The ThDP, also known as thiamine pyrophosphate (TPP) or cocarboxylase, is catalyzed by an enzyme called thiamine diphosphokinase according to the reaction thiamine + ATP → ThDP + AMP (EC 2.7.6.2). ThDP is a coenzyme for several enzymes that catalyze the transfer of two-carbon units and in particular the dehydrogenation (decarboxylation and subsequent conjugation with coenzyme A) of 2-oxoacids (alpha-keto acids). The enzymes transketolase, pyruvate dehydrogenase (PDH), and 2-oxoglutarate dehydrogenase (OGDH) are all important in carbohydrate metabolism. The cytosolic enzyme transketolase is a key player in the pentose phosphate pathway, a major route for the biosynthesis of the pentose sugars deoxyribose and ribose. The mitochondrial PDH and OGDH are part of biochemical pathways that result in the generation of adenosine triphosphate (ATP), which is a major form of energy for the cell. PDH links glycolysis to the citric acid cycle, while the reaction catalyzed by OGDH is a rate-limiting step in the citric acid cycle. In the nervous system, PDH is also involved in the production of acetylcholine, a neurotransmitter, and for myelin synthesis.

93. A 47-year-old female is brought to the emergency department with complaints of malaise, nausea and vomiting, and fatigue. The patient reveals a long history of alcohol abuse for the last 10 years requiring drinks daily especially in the morning as an “eye opener.” She has been to rehab on several occasions for alcoholism but has not been able to stop drinking. She is currently homeless and jobless. She denies cough, fever, chills, upper respiratory symptoms, sick contacts, recent travel, hematemesis, or abdominal pain. She reports feeling hungry and has not eaten very well in a long time. On physical exam she appears malnourished but in no distress. Her physical exam is normal. Her blood count reveals a normal white blood cell count but does show an anemia with large red blood cells. Her amylase, lipase, and liver function tests were normal. What is the most likely cause of her anemia? What is the molecular basis for the large erythrocytes?

Answer: Cause of anemia of this 47-year-old alcoholic female is folic acid deficiency. Abnormal proliferation of erythroid precursors in the bone marrow, since folate deficiency encumbers the maturation of these cells by inhibition of deoxyribonucleic acid (DNA) synthesis. Folate (folic acid) is an essential vitamin which, in its active form of tetrahydrofolate (THF), transfers 1-carbon groups to intermediates in metabolism. Folate plays an important role in DNA synthesis. It is required for the de novo synthesis of purines and for the conversion of deoxyuridine 5-monophosphate (dUMP) to deoxythymidine 5-monophosphate (dTMP). Additionally, folate derivatives participate in the biosynthesis of choline, serine, glycine, and methionine. However, in situations of folate deficiency, symptoms are not observed from the lack of these products as adequate levels of choline and amino acids are obtained from the diet. Folate deficiency results in megaloblastic anemia. Megaloblastic anemia is characterized by macrocytic erythrocytes produced by abnormal proliferation of erythroid precursors in the bone marrow. Folate deficiency encumbers the maturation of these cells by inhibition of DNA synthesis. Without an adequate supply of folate, DNA synthesis is limited by decreased purine and dTMP levels. Folate deficiency perturbs DNA metabolism and methylation reactions. In cases of chronic alcoholism, folate deficiency may result from poor nutrition or from poor absorption of folate secondary to a conjugase deficiency. Once folate deficiency occurs, abnormal megaloblastic replication of epithelial mucosa can occur, which further impairs folate uptake.

94. Because of the close interrelationship between the vitamins, patients with deficiencies of either folate or vitamin B12 exhibit similar symptoms. Which of the following tests would best help distinguish between a folate and vitamin B₁₂ deficiency?

Answer: Vitamin B₁₂ is a cofactor in two biochemical reactions, the conversion of homocysteine to methionine by the enzyme methionine synthase and the conversion of L-methylmalonyl-CoA to succinyl- CoA by methylmalonyl-CoA mutase. N5-methyl THF is a methyl donor in the methionine synthase reaction. A folate deficiency would result in decreased methionine synthase activity and decreases in methionine and cystathionine concentrations, while homocysteine levels would be increased. A vitamin B₁₂ deficiency would also yield these same results, but in addition methylmalonate levels would increase as a consequence of a decrease in the activity of methylmalonyl-CoA mutase activity.

95. A muscular 25-year-old male presents with dermatitis and an inflamed tongue. A history reveals that he has been consuming raw eggs as part of his training regimen for the past 6 months. Select the vitamin that is most likely to be deficient.

Answer: Raw eggs contain a protein, avidin, which binds biotin strongly. Because native avidin is resistant to hydrolysis by digestive proteases,

when it binds biotin it prevents its absorption. Avidin that has been denatured by cooking will be broken down during the digestive process. A biotin deficiency manifests itself as an erythematous, scaly skin eruption and can also cause hair loss and conjunctivitis. A biotin deficiency can also occur following prolonged total parenteral nutrition if biotin is not supplemented.

96. A 30-year-old male goes to his dentist complaining of loosening teeth. Examination also reveals his gums are swollen, purple, and spongy. The dentist also notes that the patient’s fingers have multiple splinter hemorrhages near the distal ends of the nail and that a wound on the patient’s forearm has failed to heal properly. Select the vitamin that is most likely to be deficient.

Answer: The patient exhibits the classic symptoms of scurvy, a deficiency in vitamin C. In addition to being an important biological antioxidant, ascorbic acid is required for the hydroxylation of proline and lysine residues of procollagen in the synthesis of collagen. A deficiency leads to defects in collagen synthesis, which adversely affects the intercellular cement substances in connective tissue, bones, and dentin.

97. A 59-year-old male is brought to the emergency department by the EMS after a family member found him extremely confused and disoriented, with an unsteady gait and strange irregular eye movements. The patient has been known in the past to be a heavy drinker. He has no known medical problems and denies any other drug usage. On examination, he is afebrile with a pulse of 110 beats per minute and a normal blood pressure. He is extremely disoriented and agitated. Horizontal rapid eye movement on lateral gaze is noted bilaterally. His gait is very unsteady. The remainder of his examination is normal. The urine drug screen was negative and he had a positive blood alcohol level. The emergency room physician administers thiamine. What is the most likely diagnosis? What is importance of thiamine in biochemical reactions?

Answer: Most likely diagnosis in this case is Wernicke-Korsakoff syndrome (thiamine deficiency) often associated with chronic alcoholics. Thiamine deficiency is uncommon except in alcoholics as a result of nutritional deficiencies and malabsorption. The classic clinical triad of dementia, ataxia (difficulty with walking), and eye findings may be seen, but more commonly, only forgetfulness is noted. Sometimes, thiamine deficiency can lead to vague symptoms such as leg numbness or tingling. Because thiamine is water soluble, it can be added to intravenous fluids and administered in that way. Other manifestations include beri beri, which is cardiac involvement leading to a high cardiac output, and vasodilation. Affected patients often feel warm and flushed, and they can have heart failure.

98. A full-term female infant failed to gain weight and showed metabolic acidosis in the neonatal period. A physical examination at 6 months showed failure to thrive, hypotonia, small muscle mass, severe head lag, and a persistent acidosis (pH 7.0 to 7.2). Blood lactate, pyruvate, and alanine were greatly elevated. Treatment with thiamine did not alleviate the lactic acidosis. Which of the following enzymes is most likely deficient in this patient?

Answer: The increased concentrations of pyruvate, lactate, and alanine indicate that there is a block in the pathway leading from pyruvate toward the TCA cycle. A deficiency in pyruvate dehydrogenase would lead to a buildup of pyruvate. Pyruvate has three fates other than conversion to acetyl-CoA by pyruvate dehydrogenase: conversion to oxaloacetate by pyruvate carboxylase, reduction to lactate by lactate dehydrogenase, and transamination to the amino acid alanine. Thus, because pyruvate builds up, an increase in lactate and alanine would be expected if pyruvate dehydrogenase was deficient.

99. A 3-month-old male infant developed seizures and progressively worsened, showing hypotonia, psychomotor retardation, and poor head control. He had lactic acidosis and an elevated plasma pyruvate level, both more than seven times the normal amount. Pyruvate carboxylase activity was measured using extracts of fibroblasts and was found to be less than 1 percent of the normal level. Oral administration of which of the following amino acids would you recommend as the best therapy for this patient?

Answer: A deficiency in pyruvate carboxylase results in a diminution of oxaloacetate, the C₄ acid that acts as the acceptor for an acetyl group from acetyl-CoA. In order for the TCA cycle to continue efficiently, C₄ acids must be replenished. Amino acids whose carbon skeletons feed into the TCA cycle and increase the C₄ pool will accomplish this. Glutamine, which is converted to α-ketoglutarate, will lead to an increase in all of the C₄ acids (succinate, fumarate, malate, and oxaloacetate). Alanine and serine are converted to pyruvate, which as a result of the deficiency in pyruvate carboxylase will not increase the C₄ pool. Lysine and leucine are ketogenic amino acids and thus also do not increase the C₄ pool.