We developed a promising candidate for a hypersensitive magnetic resonance (MR) reporter protein. The reporter protein is comprised of mouse lactate dehydrogenase-X (mLDH-X) and a hyperpolarized pyruvic acid derivative, substrate Y that gives no false-positive signals by endogenous human lactate dehydrogenases (hLDHs). The X–Y pair shows high MR sensitivity and sufficient reaction kinetics, thus enabling direct detection of reporter protein X in crude biological mixtures by monitoring the ¹³C NMR signal of hyperpolarized product Y^r.