31P‐MRS of healthy human brain: Measurement of guanosine diphosphate mannose at 7 T

Guanosine diphosphate mannose (GDP‐Man) is the donor substrate required for mannosylation in the synthesis of glycoproteins, glycolipids and the newly discovered glycoRNA. Normal GDP‐Man biosynthesis plays a crucial role in support of a variety of cellular functions, including cell recognition, cell communication and immune responses against viruses. Here, we report the detection of GDP‐Man in human brain for the first time, using 31 P MRS at 7 T. The presence of GDP‐Man is evidenced by the detection of a weak 31 P doublet at −10.7 ppm that can be assigned to the phosphomannosyl group (Pβ) of the GDP‐Man molecule. This weak but well‐resolved signal lies 0.9 ppm upfield of UDP(G) Pβ‐multiplet from a mixture of UDP‐Glc, UDP‐Gal, UDP‐GlcNAc and UDP‐GalNAc. In reference to ATP (2.8 mM), the concentration of GDP‐Man in human brain was estimated to be 0.02 ± 0.01 mM, about 15‐fold lower than the total concentration of UDP(G) (0.30 ± 0.04, N = 17) and consistent with previous reports of UDP‐Man in cells and brain tissue extracts measured by high‐performance liquid chromatography. The reproducibility of the measured GDP‐Man between test and 2‐week retest was 21% ± 15% compared with 5% ± 4% for UDP(G) (N = 7). The measured concentrations of GDP‐Man and UDP(G) are linearly correlated ([UDP(G)] = 4.3 [GDP‐Man] + 0.02, with R = 0.66 and p = 0.0043), likely reflecting the effect of shared sugar precursors, which may vary among individuals in response to variation in nutritional intake and consumption. Given that GDP‐Man has another set of doublet (Pα) at −8.3 ppm that overlaps with NAD(H) and UDP(G)‐Pα signals, the amount of GDP‐Man could potentially interfere with the deconvolution of these mixed signals in composition analysis. Importantly, this new finding may be useful in advancing our understanding of glycosylation and its role in the development of cancer, as well as infectious and neurodegenerative diseases.