Efficient nitrogen-13 radiochemistry catalyzed by a highly stable immobilized biocatalyst

Abstract

The continuous progress of nuclear imaging techniques demands the development and implementation of efficient, clean and sustainable synthetic routes for the preparation of novel radiotracers. This is especially challenging in the context of radiotracers labelled with short-lived positron emitters such as nitrogen-13. Biocatalysis can offer attractive alternatives to conventional chemistry, because enzymes present exquisite chemical selectivity and high turnover numbers. However, enzymes have been poorly exploited in the area of radiochemistry. Herein, we present the design and fabrication of a heterogeneous biocatalyst suitable for the reduction of [¹³N]NO₃⁻ to [¹³N]NO₂⁻. A eukaryotic nitrate reductase from Aspergillus niger was immobilized on different carriers and immobilization parameters were determined. Optimal results were obtained for agarose beads activated with a positively charged tertiary amino group (Ag-DEAE). The immobilized preparation was 12-fold more thermostable than the soluble enzyme. Biochemical characterization of the immobilized enzyme showed interesting thermally-induced hyperactivation driven by the interaction between the enzyme and the carrier. The heterogeneous biocatalyst could be re-used up to 7 reaction cycles while preserving its initial activity. Finally, to demonstrate the potential of this heterogeneous biocatalyst in the context of radiochemistry, radiosynthesis of S-[¹³N]nitrosoglutathione was carried out using the enzymatically produced [¹³N]NO₂⁻ as the labelling agent.