E. Kustermann et al., ASCORBIC-ACID, A VITAMIN, IS OBSERVED BY IN-VIVO C-13 NUCLEAR-MAGNETIC-RESONANCE SPECTROSCOPY OF RAT-LIVER, American journal of physiology: endocrinology and metabolism, 37(1), 1998, pp. 65-71
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.