Selective observation of biologically important 15N-labeled metabolites in isolated rat brain and liver by 1H-detected multiple-quantum-coherence spectroscopy

Abstract Four cerebral metabolites of importance in neurotransmission, serotonin, l -tryptophan, l -glutamine, and N -acetyl- l -aspartate, and two hepatic urea-cycle intermediates, citrulline and urea, were found to be observable by 1 H- 15 N heteronuclear multiple-quantum-coherence (HMQC) spectroscopy in aqueous solution at physiological pH and temperature, through the protons spin-coupled to their indole, amide, or ureido nitrogen. Their 1 H chemical shifts were well dispersed over a 5–10 ppm region while the 1 J 15 N- 1 H values were 87–99 Hz. For [γ- 15 N]glutamine, a 50− to 100-fold increase in sensitivity over direct 15 N detection was achieved, in contrast to a 2-fold increase by the polarization-transfer method. In the isolated brain of portacaval-shunted rats, the amide protons of biologically 15 N-enriched [γ- 15 N]glutamine were observed in 2 min of acquisition, with suppression of proton signals from all other cerebral metabolites. In isolated liver of 15 N-enriched control rats, [ 15 NIurea protons were observed in 16 min. The HMQC method is likely to be effective for the in vivo study of cerebral and hepatic nitrogen metabolism.