Here we perform the enzymatic reaction in D2O to produce [3,O-D2]malate (see Fig. ), which will henceforth be referred to as malate-D2 for simplicity. The deuterons are weakly coupled to the protons and are unaffected by the proton rf pulses, so can be ignored when we consider the spin dynamics. This means we can treat the two protons originating from parahydrogen as an isolated two-spin system, which is convenient for this proof-of-principle demonstration. Using D2O as the solvent has the additional and important benefit of extending the fumarate proton 𝑇 S , which was measured to be 8 s in H2O (see Supporting Information), but is over 40 s in D2O .