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Earth-field MRI can provide new contrasts leading to the observation of pathologies at the biochemical level. However detection sensitivity is poor at low-field. In a preliminary spectroscopic approach, it is proposed here to detect protease-driven hydrolysis of a nitroxide probe thanks to electron-nucleus Overhauser enhancement in a homemade double resonance system. The nitroxide probe is a six-line nitroxide whose lines are shifted according to its substrate/product state. The Overhauser enhancement frequency dependence was in agreement with theoretical calculations. Enzymatic conversion of the nitroxide substrate was observed which opens the way for the design of new low-field DNP-MRI systems.