An obese patient had failed to lose weight because he could not follow prescribe

Question

An obese patient had failed to lose weight because he could not follow prescribed weight reduction diets. Consequently, he volunteered to go on a starvation diet as part of a study of amino acid metabolism. Blood samples were taken and analyzed for plasma amino acids during a 6 week fast. Valine, leucine and isoleucine were transiently increased during the first week but dropped below initial levels at later times. A total of 13 amino acids ultimately decreased in plasma at the end of the study. The largest decrease was seen for alanine, which rapidly dropped 70% during the first week. Total plasma amino nitrogen dropped only 12%.

Explain how the decreased plasma alanine concentration may be related to gluconeogenesis.

The branched-chain amino acids stimulate the production of alanine and glutamate in muscle. Explain this observation.

Explain why patients with maple syrup urine disease suffer hypoglycemic episodes even though insulin levels are normal. Consider the fact that plasma alanine levels in these patients are 3 to 10-fold below normal.

Explanation / Answer

Alanine and glutamine are amino acid precursors of gluconeogenesis. So, when the blood glucose levels are low, body stimulates the synthesis of glucose from non-carbohydrate sources like amino acids.

Alanine production in muscle appears related to the rapid oxidation of branched chain amino acids.

Hypoglycemia associated with Maple syrup urine disease appears to be related to a defect in gluconeogenesis from amino acids, which cannot be accounted for by abnormalities of the rate limiting gluconeogenic enzymes. It is detected that 3-carbon substrates are shunted from amino acids into glutamine leading to decreased production of net oxaloacetate and impaired gluconeogenesis.

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