Whole organ metabolism studied by NMR.

Nuclear magnetic resonance (NMR) is now established as a noninvasive method of studying metabolism in vivo, and in this review we discuss the contribution that this technique can make to our understandin� of cellular metabolism. Since 1979, in vivo NMR has been the subject of a large number of reviews (18, 39-41, 44, 46, 55, 56, 63, 88, 90, 96, 97, 99), and in order to avoid excessive overlap this review concentrates on the most recent developments that have taken place. There have been some notable advances during the last two or three years, and the most eye-catching probably has been the application of NMR to human metabolism. The emergence of a new technique for studying metabolism inevitably invites comparison with the more traditional methods that are available. The main feature ofNMR is that it is nondestructive and noninvasive, and so metabolic processes can be followed as they take place within a living system. In addition, certain aspects of the intracellular environment can be monitored; for example, the intracellular pH is commonly measured by 31p NMR using the frequency dependence of the inorganic phosphate signal. Furthermore, it is possible to study reactions taking place in vivo under steady state or equilibrium conditions. The main disadvantage of the technique is that it is insensitive, and therefore it can directly detect only those metabolites that are present at concentrations of about 0.2-0.5 mM or above. For this and a variety of other reasons, NMR should be regarded as a technique that is complementary to, rather than com­ peting with, the other methods that are available for studying metabolism. The majority of metabolic NMR studies have focused on the 31 P nucleus. Spectra can be obtained reasonably rapidly (typically in about a minute), they can be interpreted very readily, and they are highly informative; often