Abstract
Dimeric indole alkaloids (DIAs), such as vinblastine and vincristine, found in Catharanthus roseus are used clinically as antitumor drugs. A stable supply of DIAs is desired because these alkaloids are very expensive due to their low abundance in plants. A coupling reaction between catharanthine (CAT) and vindoline (VID) is the rate-limiting step of DIAs biosynthesis in planta. 3', 4'-Anhydrovinblastine (AVLB), the product of the coupling reaction, is the precursor of CAT and VID. Therefore, an effective AVLB production system is greatly required. Previously we found that the coupling reaction of CAT and VID to produce AVLB occurred in the presence of flavin mononucleotide and manganese ion (II) by irradiation with near-ultraviolet light at a peak of 370 nm without the presence of any enzyme. In this study, we investigated the effects of organic solvents on this non-enzymatic reaction. We show that the addition of 10% methanol to the reaction mixture permitted the preparation of a highly concentrated substrate solution, resulting in a high yield of AVLB by the coupling reaction. Conditions for the coupling reaction in 10% methanol solution were optimized. We also confirmed that the coupling reaction could occur in crude extracts of C. roseus obtained by organic solvent extraction. These findings suggest a method to produce DIAs on a large scale with reduced production costs.