A 31-year-old previously healthy man comes to the office due to myalgias, anorexia, and skin rash. He does not use tobacco, alcohol, or illicit drugs. The patient works as a personal trainer and is a bodybuilding enthusiast. He denies using anabolic steroids but has been consuming large amounts of raw egg whites for the past several months. Physical examination shows macular dermatitis of the extremities. A water-soluble vitamin deficiency is suspected as the cause of his condition. Which of the following biochemical conversions most likely uses the deficient vitamin as a cofactor?

Question

A 31-year-old previously healthy man comes to the office due to myalgias, anorexia, and skin rash.  He does not use tobacco, alcohol, or illicit drugs.  The patient works as a personal trainer and is a bodybuilding enthusiast.  He denies using anabolic steroids but has been consuming large amounts of raw egg whites for the past several months.  Physical examination shows macular dermatitis of the extremities.  A water-soluble vitamin deficiency is suspected as the cause of his condition.  Which of the following biochemical conversions most likely uses the deficient vitamin as a cofactor?

Quizplus · Accepted Answer

11ec160b_36d7_186f_9249_fb6e80c4f52d_MD0001_00 Biotin (vitamin B₇) is an important cofactor for several carboxylase enzymes. It functions as a CO₂ carrier and plays an essential role in carbohydrate, lipid, and amino acid metabolism. In the liver, the conversion of pyruvate to oxaloacetate for gluconeogenesis requires pyruvate carboxylase and biotin. Biotin deficiency is rare but can occur secondary to poor diet, excessive raw egg white consumption (due to high levels of biotin-binding avidin in egg whites), and congenital disorders of biotin metabolism. Patients with biotin deficiency present with nonspecific symptoms, including changes in mental status, myalgias, anorexia, and chronic dermatologic changes such as macular dermatitis. Biotin-deficient individuals can also develop metabolic acidosis as a result of increased conversion of pyruvate to lactic acid. (Choice A) Niacin (vitamin B₃), in the form of NADP⁺, is used to oxidize glucose-6-phosphate in the pentose phosphate pathway, generating ribose-5-phosphate and NADPH in the process. (Choice B) Thiamine (vitamin B₁), riboflavin (vitamin B₂), niacin (vitamin B₃), pantothenic acid (vitamin B₅), and lipoic acid are the 5 cofactors required by the mitochondrial enzyme complex pyruvate dehydrogenase. In their absence, the metabolism of pyruvate to acetyl-CoA is not possible. (Choice C) Pyridoxal phosphate (vitamin B₆) acts as a cofactor for alanine transaminase, which catalyzes the reversible reaction between pyruvate and glutamate to alanine and alpha-ketoglutarate in the liver and muscle tissue. (Choice E) Production of oxaloacetate from succinate in the TCA cycle requires 2 enzymes (succinate dehydrogenase and malate dehydrogenase), along with riboflavin (vitamin B₂) and niacin (vitamin B₃) in the forms of cofactors FAD⁺ and NAD⁺, respectively. Educational objective: Biotin acts as a CO₂ carrier on the surface of carboxylase enzymes and is an essential cofactor for numerous reactions, including the conversion of pyruvate to oxaloacetate and fatty acid metabolism. Excess ingestion of avidin, found in egg whites, has been associated with biotin deficiency. This condition presents with mental status changes, myalgias, anorexia, macular dermatitis, and lactic acidosis.

A 31-Year-Old Previously Healthy Man Comes to the Office Due