Intracellular redox state revealed by in vivo ³¹P MRS measurement of NAD⁺ and NADH contents in brains

Purpose: Nicotinamide adenine dinucleotide (NAD), in oxidized (NAD ⁺) or reduced (NADH) form, plays key roles in cellular metabolism. Intracellular NAD⁺/NADH ratio represents the cellular redox state; however, it is difficult to measure in vivo. We report here a novel in vivo ³¹P MRS method for noninvasive measurement of intracellular NAD concentrations and NAD⁺/NADH ratio in the brain. Methods: It uses a theoretical model to describe the NAD spectral patterns at a given field for quantification. Standard NAD solutions and independent cat brain measurements at 9.4 T and 16.4 T were used to evaluate this method. We also measured T ₁ values of brain NAD. Results: Model simulation and studies of solutions and brains indicate that the proposed method can quantify submillimolar NAD concentrations with reasonable accuracy if adequate ³¹P MRS signal-to-noise ratio and linewidth were obtained. The NAD concentrations and NAD⁺/NADH ratio of cat brains measured at 16.4 T and 9.4 T were consistent despite the significantly different T₁ values and NAD spectra patterns at two fields. Conclusion: This newly established ³¹P MRS method makes it possible for the first time to noninvasively study the intracellular redox state and its roles in brain functions and diseases, and it can potentially be applied to other organs.