Characterization of amine proton exchange for analyzing the specificity and intensity of the CEST effect: from humans to fish.

Felizitas C Wermter,Wolfgang Dreher,Christian Bock

doi:10.1002/nbm.4622

Abstract

Chemical exchange saturation transfer (CEST) at about 2.8 ppm downfield from water is characterized besides other compounds by exchanging amine protons of relatively high concentration amino acids and is determined by several physiological (pH, T) and experimental (B0 , B1 , tsat ) parameters. Although the weighting of the CEST effect observed in vivo can be attributed mainly to one compound depending on the organism and organ, there are still several other amino acids, proteins and molecules that also contribute. These contributions in turn exhibit dependences and thus can lead to possible misinterpretation of the measured changes in the CEST effect. With this in mind, this work aimed to determine the exchange rates of six important amino acids as a function of pH and temperature, and thus to create multi-pool models that allow the accurate analysis of the CEST effect concerning different physiological and experimental parameters for a wide variety of organisms. The results show that small changes in the above parameters have a significant impact on the CEST effect at about 2.8 ppm for the chosen organisms, i.e. the human brain (37 °C) and the brain of polar cod (1.5 °C), furthermore, the specificity of the CEST effect observed in vivo can be significantly affected. Based on the exchange rates ksw (pH, T) determined for six metabolites in this study, it is possible to optimize the intensity and the specificity for the CEST effect of amino acids at about 2.8 ppm for different organisms with their specific physiological characteristics. By adjusting experimental parameters accordingly, this optimization will help to avoid possible misinterpretations of CEST measurements. Furthermore, the multi-pool models can be utilized to further optimize the saturation.

Highlights

Chemical exchange saturation transfer (CEST) imaging enables the indirect detection of endogenous and exogenous compounds such as amino acids, proteins and other molecules with exchangeable protons exhibiting, e.g., amide (-NH), amine (-NH2) or hydroxyl (-OH) groups
Other important metabolites involved in brain metabolism, such as glucose (-OH, 1.2 ppm),[48] creatine (À[NH2]2+, 1.9 ppm)[49] and myo-inositol (-OH, 0.6 ppm),[50] can be neglected in this consideration, as their effects are almost not involved in the total CEST effect at about 2.8 ppm for a physiological pH of 7.2
The validity of these approaches is limited to rather slow exchange rates that fulfill the conditions ksw ( ω1 and ksw ( Δω, as they apply to the amide protons of, e.g., poly-L-lysine (PLL), but not to the amine protons investigated here, which are characterized by faster exchange rates.[11]

Summary

Introduction

Chemical exchange saturation transfer (CEST) imaging enables the indirect detection of endogenous and exogenous compounds such as amino acids, proteins and other molecules with exchangeable protons exhibiting, e.g., amide (-NH), amine (-NH2) or hydroxyl (-OH) groups. The CEST effect, being observed after saturation at about 2.8 ppm downfield from water is, among other things, largely determined by exchangeable amine protons of relatively highly concentrated amino acids, which play an important role in the metabolism of the central nervous system of vertebrates and fulfil a variety of specific functions.[3] For example, they form the basis for the synthesis of proteins and nucleic acids or are involved in energy metabolism and neurotransmission. Their non-specific functions include, e.g., their involvement in osmoregulation.

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