The conversion of littorine to hyoscyamine has been investigated by feeding deuterium labelled (RS)-[2-H-2]-, [3, 3-H-2(2)]-, [2, 3, 3-H-2(3)]- phenyllactic acids to transformed root cultures of Datura stramonium. Isolation and GC-MS analyses of the isotope incorporation into the resultant hyoscyamine does not support the involvement of a vicinal interchange process operating during the isomerisation of littorine to hyoscyamine. Additionally a metabolism study with [1′-C-13, 3′, 3′-H-2(2)]-hyoscyamine has established that the alkaloid is metabolically stable at C-3′ with no evidence for a reversible in vivo oxidation process to the corresponding aldehyde. The data do not support an S-adenosy-L-methionine (SAM 5)/co-enzyme-B-12, mediated process for the isomerisation of littorine to hyoscyamine. (C) 2002 Elsevier Science Ltd. All rights reserved.