ASCORBIC-ACID, A VITAMIN, IS OBSERVED BY IN-VIVO C-13 NUCLEAR-MAGNETIC-RESONANCE SPECTROSCOPY OF RAT-LIVER

The first in vivo detection of a vitamin with nuclear magnetic resonan ce (NMR) is reported for mammalian liver. Vitamin C, also known as asc orbic acid, was monitored noninvasively in rat liver by ''whole body'' C-13 NMR spectroscopy at high field after infusion of [1,2-C-13(2)]gl ucose into anesthetized rats. Generally, the carbon resonances of asco rbic acid overlap with those of other highly abundant cellular metabol ites, thus precluding their observation in situ. This problem was reso lved by taking advantage of the C-13-C-13 spin couplings introduced by the two covalently bound C-13 nuclei in [1,2-C-13(2)]glucose. During glucose metabolism, [5,6-C-13(2)]ascorbic acid was synthesized, which also exhibited characteristic C-13 homonuclear spin couplings. This fe ature enabled the spectral discrimination of ascorbic acid from overla pping singlet resonances of other metabolites. Quantitative analysis o f the spin-coupling patterns provided an estimate of the turnover rate of hepatic ascorbic acid in vivo (1.9 +/- 0.4 nmol . min(-1) . g(-1)) and a novel approach toward a better understanding of optimal ascorbi c acid requirements in humans. The results obtained in vivo were confi rmed with high-resolution proton and C-13 NMR spectroscopy of liver ex tracts.