The enzyme norcoclaurine synthase (NCS) found in the common meadow rue, Thalictrum flavum, and other plants shows sequence homology to members of the class 10 of pathogenesis related (PR 10) proteins that contains allergens such as the major birch pollen allergen Bet v 1, the major cherry allergen Pru av 1, and the major apple allergen Mal d 1. The enzyme is involved in the plant's secondary metabolism and is required for the production of bioactive secondary metabolites like morphine. Whereas the physiological function of PR 10 class allergens is still unknown, NCS activity has been studied in detail. Investigation of the structural properties of NCS by NMR spectroscopy can thus not only provide new information concerning the reaction mechanism of the enzyme, but is also expected to help clarify the long standing and heavily debated question on the physiological function as well as the reasons for the allergenic potential of members of this protein family. As the first important step towards the three-dimensional solution structure, we optimized expression of recombinant NCS in Escherichia coli and established an efficient purification protocol yielding high amounts of pure isotopically labeled active enzyme. The identity of NCS was confirmed by electrospray ionization mass spectrometry, and activity of the purified enzyme was determined by an assay detecting the radiolabeled reaction product. Spectroscopic analysis by NMR spectroscopy showed that the protein was properly folded with well defined tertiary structure. (C) 2007 Elsevier Inc. All rights reserved.