Updated: 7/13/2019

Amino Acid Absorption

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Overview
  •  Nitrogen balance 
    • positive
      • amino acid (AA) intake > excretion
      • examples
        • growth
        • pregnancy
    • negative
      • AA excretion > intake
      • examples
        • kwashiorkor
        • starvation
        • infection
  • AA absorption in intestine
    • proteins degraded by trypsin and pepsin
      • enteropeptidase cleaves trypsinogen (inactive) to trypsin (active)
    • absorbed as AA's across the gut lumen with specific transporter for similar AA's
      • e.g., transporter for basic AA's, transporter for large neutral AA's
    • deficiencies
      • lack of absorption with pancreatitis due to ↓ in degradative enzyme production
      • Hartnup disease
        • defect in large neutral AA transporter in the intestine and renal proximal tubule cells
        • result is a tryptophan deficiency
        • presents similar to pellagra
  • AA absorption in kidney
    • AA's that are filtered from the glomerulus can be actively reabsorbed in the proximal convoluted tubules with similar transporters as the gut
    • deficiencies
      • cystinuria
        • genetic defect in transporter for cysteine, ornithine, lysine, arginine 
        • AR
        • presentation
          • cystine staghorn calculi
            • note: cystine = 2x cysteine attached by a disulfide bridge
            • cystine kidney stones result from high concentrations in urine
        • treatment
          • alkalinization of the urine with acetazolamide

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Questions (2)
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(M1.BC.15.74) A 12-year-old boy presents to the pediatrician for the dermatological changes seen in Figure A that began two weeks ago after he begun his first day of football practice. The skin eruptions appeared to be localized solely to sun exposed areas. The boy notes that he has also had diarrhea, headaches, and confusion appearing at the same time. On examination, the pediatrician notes ataxia and tremors in the boy. A urinalysis revealed elevated levels of tyrosine, leucine, valine, isoleucine, tryptophan, glycine, alanine, and phenylalanine. Which of the following vitamin deficiencies would produce a similar clinical presentation?

QID: 106622
FIGURES:
1

Thiamine

13%

(29/225)

2

Riboflavin

1%

(3/225)

3

Niacin

73%

(164/225)

4

Pyridoxine

8%

(19/225)

5

Cobalamin

3%

(7/225)

M 1 D

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