¹³C saturation transfer effect of carbon dioxide–bicarbonate exchange catalyzed by carbonic anhydrase in vivo

Abstract

Carbonic anhydrase catalyzes reversible hydration of carbon dioxide and dehydration of bicarbonate. In this article we report that the rapid exchange catalyzed by carbonic anhydrase causes a large magnetization (saturation) transfer effect on the 13 C signal of bicarbonate at 160.7 ppm in vivo when the resonance of the undetectable carbon dioxide at 125.0 ppm is irradiated with RF pulses. In isoflurane-anesthetized adult rat brain the unidirectional, pseudo first-order rate constant of this exchange in the dehydration direction was determined to be 0.47 ؎ 0.05 sec ؊1 following intravenous infusion of uniformly 13 C-labeled glucose for labeling bicarbonate. Intralateral ventricular administration of the highly specific carbonic anhydrase inhibitor acetazolamide, which is a drug used for treating glaucoma and epilepsy, was also shown to significantly attenuate the observed 13 C magnetization transfer effect of the carbon dioxide-bicarbonate exchange in the rat brain. Magn Reson Med 59:492-498, 2008.

Studying enzymes by in vivo 13C magnetic resonance spectroscopy

“…In our study [27], we measured the in vivo 13 C magnetization transfer effect of the CA-catalyzed carbon dioxide-bicarbonate reaction, quantified the pseudo-first-order unidirectional bicarbonate → carbon dioxide rate constant, and demonstrated the effect of acetazolamide treatment on 13 C magnetization transfer between carbon dioxide and bicarbonate.…”

“…The in vivo 13 C T 1 of bicarbonate was determined to be 11.8 ± 0.4 s −1 (mean ± SD). The pseudo-first-order unidirectional bicarbonate → carbon dioxide rate constants $k_{{\text{italicHCO}}_3^{-}\to {\text{italicCO}}_2}$ were calculated to be 0.47 ± 0.05 s −1 (mean ± SD) for the control brain, and 0.073 ± 0.007 s −1 (mean ± SD) for the acetazolamide-treated brain [27]. …”

“…The exchange of bicarbonate across the membrane of packed erythrocytes was investigated using radiofrequency saturation of carbon dioxide and inclusion of relevant transmembrane transport terms into the Bloch-McConnell equations [59]. We found that the combination of relatively long 13 C T 1 of bicarbonate and fast exchange between carbon dioxide and bicarbonate catalyzed by CA causes a large and quantifiable 13 C magnetization effect in vivo [27]. Although only the cerebral carbon dioxide ↔ bicarbonate exchange catalyzed by CA was investigated, we expect that this technique can also be used to study carbonic anhydrase-catalyzed carbon dioxide ↔ bicarbonate exchange in other organs or tissues because of the wide distribution of carbonic anhydrase, carbon dioxide, and bicarbonate [151].…”

“…We hypothesized that if this exchange rate is very rapid it should be directly measurable using magnetization transfer. This line of research first led to the discovery of the in vivo magnetization transfer effect catalyzed by aspartate aminotransferase (AAT), then by lactate dehydrogenase (LDH) [25], malate dehydrogenase (MDH) [26], and carbonic anhydrase (CA) [27]. We demonstrated that the chemical exchange processes of these enzymes could be measured by 13 C saturation transfer with 13 C detection [24–27] and/or 13 C saturation transfer with 1 H detection techniques [28].…”

“…This line of research first led to the discovery of the in vivo magnetization transfer effect catalyzed by aspartate aminotransferase (AAT), then by lactate dehydrogenase (LDH) [25], malate dehydrogenase (MDH) [26], and carbonic anhydrase (CA) [27]. We demonstrated that the chemical exchange processes of these enzymes could be measured by 13 C saturation transfer with 13 C detection [24–27] and/or 13 C saturation transfer with 1 H detection techniques [28]. We also found that the exchange between 13 C-labeled mitochondrial and cytosolic pools in brain is much faster than the tricarboxylic acid (TCA) cycle flux [29].…”