A sample of hyperpolarized fumarate was generated as previously described, but with 20% [1- C] isotopic enrichment of the starting material. After sample precipitation, the fumarate was redissolved in a phosphate buffer, and the resulting solution was mixed with fumarase enzyme. The 13 C NMR signals were acquired every 7 s using 15° flip angle pulses to monitor the enzymecatalyzed conversion of fumarate to malate, and the results are shown in Fig. . Further experimental details are given in the Materials and Methods section. The hyperpolarized [1- C]fumarate signal at 175.4 ppm can be seen to decay, and two additional resonances corresponding to [1- C]malate and [4- C]malate appear at 181.8 and 180.6 ppm, respectively. At first the malate signals grow in intensity as the initial rate of metabolism is high, and then decay predominantly due to nuclear spin relaxation.