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 st udied 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), argini ne, and proline were measured using negative chemical ionization, sele cted-ion monitoring gas chromatography-mass spectrometry. In this isot opic approach, the appearance of U-C-13-amino acid in the plasma refle cts entry of the dietary amino acids, and the appearance of C-13 in lo wer mass isotopomers demonstrates synthesis of the respective amino ac ids by the subject. All the mass isotopomers (including [M + 4]) of GL X became enriched with C-13. We suggest that the [M + 4] isotopomer of GLX reflects synthesis of the amino acid from alpha-ketoglutarate der ived from the metabolism of U-C-13 carbohydrate by the bacterial flora . Arginine labeling showed two patterns. The [M + 5] isotopomer of pla sma 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 pa ss. The [M + 1] to [M + 3] isotopomers of arginine were also labeled f or 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 asso ciated with increased labeling of the [M + 1] to [M + 3] isotopomers o f both arginine and proline, suggesting an increase in the contributio n of de novo synthesis to their plasma flux. We conclude that proline synthesis either is strictly regulated by dietary proline or is substa ntially compartmentalized. Copyright (C) 1995 by W.B. Saunders Company