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F. N., an obese patient, had failed to lose weight because he could not follow prescribed weight reduction diets. Consequently, he volunteered to go ona starvation diet as part of a study of amino acid metabolism during prolonged fasts. Blood samples were taken and analyzed for plasma amino acids during a 5- to 6-week fast. Valine, leucíne,isoleucíne, methionine, and alfa-amínobutyrate were transiently increased during the first week but dropped below initial levels at later times. Glycine, threonine, and serine increased more slowly, butthirteen amino acids ultimately decreased. The largest decrease was seen for alanine, which rapidly dropped 70% during the first week. Total plasma amino nitrogen dropped only 12%.

Biochemical questions1. Explain how the decreased plasma alanine concentration may be related
to gluconeogenesis. What is the alanine-glucose cycle?
2. Threonine, an essential amino acid, can be convertedto glycíne: thus a
block in glycine metabolism might allow both threonine and glycine to
accumulate. As a result, folic acid was administered, and the levels of glycine and threonine were measured.Although there was no change as a result of this treatment, what was the rationale for it?
3. What is the source of the plasma o-amínobutyrate?
4. What may account for the increased plasmaconcentrations of the branched chain amino acids after the first 5 days of starvation?
5. It has been shown in animals that circulating branched-chaín amino acids
are metabolized by muscle toyield energy. What are the products of the
catabolism of Ieucine, valine, and isoleucine? Is urea made in muscle
6. The branched-chaín amino acids stimulate the production of bothalanine and glutamateín animal muscle. Explain this observation, recognizing
that muscle is rich in glutamate pyruvate transaminase.
1. Explain why patients with maple syrup urine disease suffer...
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