ISOTOPIC EVIDENCE FOR THE DIFFERENTIAL REGULATION OF ARGININE AND PROLINE SYNTHESIS IN MAN

Arginine and proline derive from the metabolism of Delta(1)-pyrolline-5-carboxylate, a product of intestinal glutamic acid metabolism. We studied the extent of glutamate, arginine, and proline synthesis in four adult fed and fasted women. The subjects ingested a single dose of a mixture of uniformly (U)-C-13-labeled amino acids and carbohydrate of algal origin. Frequent blood samples were taken for 24 hours. All the mass isotopomers of plasma lysine, glutamate + glutamine (GLX), arginine, and proline were measured using negative chemical ionization, selected-ion monitoring gas chromatography-mass spectrometry. In this isotopic approach, the appearance of U-C-13-amino acid in the plasma reflects entry of the dietary amino acids, and the appearance of C-13 in lower mass isotopomers demonstrates synthesis of the respective amino acids by the subject. All the mass isotopomers (including [M + 4]) of GLX became enriched with C-13. We suggest that the [M + 4] isotopomer of GLX reflects synthesis of the amino acid from alpha-ketoglutarate derived from the metabolism of U-C-13 carbohydrate by the bacterial flora. Arginine labeling showed two patterns. The [M + 5] isotopomer of plasma arginine was labeled as rapidly as [M + 6] (ie, tracer) arginine, and we propose that the appearance of the [M + 5] isotopomer reflects the synthesis of citrulline from dietary [M + 5]-glutamate in first pass. The [M + 1] to [M + 3] isotopomers of arginine were also labeled for a prolonged period of time, suggesting that systemic glutamate was also a precursor for arginine synthesis. In fed subjects, only the [M + 5] isotopomer of proline was significantly labeled. Fasting was associated with increased labeling of the [M + 1] to [M + 3] isotopomers of both arginine and proline, suggesting an increase in the contribution of de novo synthesis to their plasma flux. We conclude that proline synthesis either is strictly regulated by dietary proline or is substantially compartmentalized. Copyright (C) 1995 by W.B. Saunders Company