Ricinine demethylation

N-Demethylation has been found to be connected with senescence of the relevant plants in the cases of nicotine (Mothes et aL, 1957), hyoscy-amine (Romeike, 1964), and ricinine (Skursky et al., 1969). Demethylation seems to have some connection with the transport of the alkaloid within the plant body (Skursky and Waller, 1972 Nowacki and Waller, 1973). In several alkaloids, e.g., nicotine (Kisaki and Tamaki, 1966), demethylation probably initiates more extensive processes of degradation of the molecule. 

A new ricinine metabolite was conclusively identified as O-demethyl-ricinine (see Figure 6.14) (Lee and Waller, 1972a) (A -methyl-3-cyano-4-hydroxy-2-pyridone), and this interconversion with ricinine in senescent and green castor plant leaves was demonstrated. [3,5- dRicinine adminis-... 

Figure 6.14. Methylation and demethylation reactions involving ricinine in castor bean plant leaves. Conversion percentages are based on the results obtained when 1 mg of precursor was administered to one leaf weighing about 5 g (Skursky et ai, 1969). Courtesy of the American Association of Biological Chemists.

Export of ricinine from senescent leaf tissues has been unequivocally established (Table 6.4) in a time course study about the accumulation of ricinine and its demethylated forms in the stems and yellow and green leaves following the administration of [3,5- C]ricinine to a yellow leaf. In... 

Table 6.4. Demethylation and Translocation of [3,5- C]Ricinine from Yellow Leaf to Green Leaf of Castor Bean Plant Cuttings ...

Figure 6.15 shows the demethylation and translocation of [3,5-ricinine administered to a yellow leaf attached to the lower part of the stem of a mature castor bean plant. The radioactivity of ricinine and the... 

Figure 6.15. Demethylation and translocation of [3,5- K ]ricinine from the yellow leaf to the adjacent green leaf of the castor bean plant cutting with one green and one yellow leaf, a— Ricinine in yellow leaves b—demethylated ricinines in yellow leaves c—ricinine in stems d—demethylated ricinines in stems e—demethylated ricinines in green leaves /—ricinine in green leaves g— green leaf h—yellow leaf (Lee and Waller, 1972). Courtesy of Peigamon Press, Ltd., copyright 1972.

Interpretations of the above two results were complicated by the fact that the radioactivity ratio between ricinine and its demethylated forms did not necessarily indicate the ratio of translocated forms since the demethylated form of ricinine upon arrival to the green leaf from the yellow leaf may undergo methylation. The result showed that ricinine administered in the yellow leaves was translocated to healthy parts of the plant, especially the growing apex where ricinine is actively synthesized. The result supports the idea that a ricinine translocation process might be a salvage operation performed by the plant in order to reutilize ricinine from the leaves which are being prepared for abscission. This is a broad concept of... 

Biotransfonnations of Alkaloids.—For a long time it was considered that alkaloids were end-products of metabolism. However, it is now being realized that many of them are rapidly metabolized, either to other alkaloids, or to non-alkaloidal compounds. Waller and co-workers have carried out extensive work in the past few years on ricinine metabolism. In senescent leaves of Ricinus communis, ricinine (18) is demethylated to N-demethylricinine (19). In... 

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