In vivo carbon-13 magnetization transfer effect. Detection of aspartate aminotransferase reaction

Abstract

One of the most remarkable achievements of in vivo NMR spectroscopy has been the detection of rapid enzyme-catalyzed exchange reactions using phosphorus-31 magnetic resonance spectroscopy-based magnetization transfer experiments. In this paper, we report, for the first time, the in vivo carbon magnetization transfer (CMT) effect and in vivo detection of the CMT effects of the α-ketoglutarate ↔ glutamate and the oxaloacetate ↔ aspartate reactions, both of which are catalyzed by aspartate aminotransferase. By saturating the carbonyl carbon of α-ketoglutarate at 206 ppm in α-chloralose anesthetized adult rat brain, the unidirectional glutamate → α-ketoglutarate flux was determined to be 78 ± 9 μ/g/min (mean ± SD, n = 11) following i.v. infusion of [1,6-13C 2]D-glucose. Contribution from aspartate aminotransferase-catalyzed partial reactions to the observed CMT effects was emphasized. Because of the large chemical shift separation between the α-carbons of the amino acids and the carbonyl carbons of the corresponding cognate keto acids, the spillover of the saturation radiofrequency pulses to the α-carbon resonances was negligible. The results indicate that the magnetization transfer effects of aspartate aminotransferase-catalyzed reactions can be used as new biomarkers accessible to non-invasive in vivo magnetic resonance spectroscopy techniques.