31P-MRS of the healthy human brain at 7 T detects multiple hexose derivatives of uridine diphosphate glucose

Nucleotide sugars are required for the synthesis of glycoproteins and glycolipids, which play crucial roles in many cellular functions such as cell communication and immune responses. Uridine diphosphate-glucose (UDP-Glc) was previously believed to be the only nucleotide sugar detectable in brain by P-31-MRS. Using spectra of high SNR and high resolution acquired at 7 T, we showed that multiple nucleotide sugars are coexistent in brain and can be measured simultaneously. In addition to UDP-Glc, these also include UDP-galactose (UDP-Gal), -N-acetyl-glucosamine (UDP-GlcNAc) and -N-acetyl-galactosamine (UDP-GalNAc), collectively denoted as UDP(G). Coexistence of these UDP(G) species is evident from a quartet-like multiplet at -9.8 ppm (M-9.8), which is a common feature seen across a wide age range (24-64 years). Lineshape fitting of M-9.8 allows an evaluation of all four UDP(G) components, which further aids in analysis of a mixed signal at -8.2 ppm (M-8.2) for deconvolution of NAD(+) and NADH. For a group of seven young healthy volunteers, the concentrations of UDP(G) species were 0.04 +/- 0.01 mM for UDP-Gal, 0.07 +/- 0.03 mM for UDP-Glc, 0.06 +/- 0.02 mM for UDP-GalNAc and 0.08 +/- 0.03 mM for UDP-GlcNA, in reference to ATP (2.8 mM). The combined concentration of all UDP(G) species (average 0.26 +/- 0.06 mM) was similar to the pooled concentration of NAD(+) and NADH (average 0.27 +/- 0.06 mM, with a NAD(+)/NADH ratio of 6.7 +/- 2.1), but slightly lower than previously found in an older cohort (0.31 mM). The in vivo NMR analysis of UDP-sugar composition is consistent with those from tissue extracts by other modalities in the literature. Given that glycosylation is dependent on the availability of nucleotide sugars, assaying multiple nucleotide sugars may provide valuable insights into potential aberrant glycosylation, which has been implicated in certain diseases such as cancer and Alzheimer's disease.