Detection of 17O-tagged phosphate by (31)P MRS: a method with potential for in vivo studies of phosphorus metabolism.

We present a method for MR detection of (17)O-labeled phosphate groups. The method employs the T(2) relaxivity effect of (17)O on (31)P nuclei to distinguish between (17)O-labeled and unlabeled phosphate groups, and uses spin-echo (SE) acquisitions with RF decoupling at the (17)O frequency to generate (31)P spectra that show only (17)O-labeled phosphate groups. The method provides an alternative to spin-labeling experiments, which are limited to the study of rapidly exchanging phosphate groups by the T(1) relaxation rates of phosphorus nuclei. We demonstrate separation of MR signals from labeled and unlabeled phosphate-containing compounds, and characterization of the T(2) effect of (17)O on phosphate nuclei in (17)O-labeled phosphate groups. Previous (17)O and (18)O phosphate-labeled studies used mass spectrometry or high-resolution MR spectroscopy (MRS) to detect the presence of an isotopic label, which requires ex vivo sample preparation. In our method the detection of (17)O-labeled phosphate is manifested as a large change in (31)P T(2), and thus allows in vivo detection using simple MR methods. Thus this method may have potential for in vivo studies of bioenergetics and the metabolism of phosphate-containing compounds.