Abstract
Nicotinamide adenine dinucleotide (NAD(+)) plays a central role in cellular metabolism both as a coenzyme for electron-transfer enzymes as well as a substrate for a wide range of metabolic pathways. In the current study NAD(+) was detected on rat brain in vivo at 11.7T by 3D localized (1)H MRS of the NAD(+) nicotinamide protons in the 8.7-9.5 ppm spectral region. Avoiding water perturbation was critical to the detection of NAD(+) as strong, possibly indirect cross-relaxation between NAD(+) and water would lead to a several-fold reduction of the NAD(+) intensity in the presence of water suppression. Water perturbation was minimized through the use of localization by adiabatic spin-echo refocusing (LASER) in combination with frequency-selective excitation. The NAD(+) concentration in the rat cerebral cortex was determined at 296 ± 28 μm, which is in good agreement with recently published (31) P NMR-based results as well as results from brain extracts in vitro (355 ± 34 μm). The T1 relaxation time constants of the NAD(+) nicotinamide protons as measured by inversion recovery were 280 ± 65 and 1136 ± 122 ms in the absence and presence of water inversion, respectively. This confirms the strong interaction between NAD(+) nicotinamide and water protons as observed during water suppression. The T2 relaxation time constants of the NAD(+) nicotinamide protons were determined at 60 ± 13 ms after confounding effects of scalar coupling evolution were taken into account. The simplicity of the MR sequence together with the robustness of NAD(+) signal detection and quantification makes the presented method a convenient choice for studies on NAD(+) metabolism and function. As the method does not critically rely on magnetic field homogeneity and spectral resolution it should find immediate applications in rodents and humans even at lower magnetic fields.