Alkaloid N-oxides as transport and vacuolar storage compounds of pyrrolizidine alkaloids inSenecio vulgarisL

✍ Scribed by Adelheid Ehmke; Kirsten Borstel; Thomas Hartmann

• Book ID: 104753082
• Publisher: Springer-Verlag
• Year: 1988
• Tongue: English
• Weight: 835 KB
• Volume: 176
• Category: Article
• ISSN: 0032-0935
• DOI: 10.1007/bf00392483

✦ Synopsis

Cell-suspension cultures of pyrrolizidinealkaloid-producing species selectively take up and accumulate senecionine (sen) and its N-oxide (sen-Nox). Cultures established from non-alkaloid-producing species are unable to accumulate the alkaloids. The uptake and accumulation of 14C-labelled alkaloids was studied using a Senecio vulgaris cell-suspension culture as well as protoplasts and vacuoles derived from it. The alkaloid uptake exhibits all characteristics of a carrier-mediated transport. The uptake of sen-Nox follows a multiphasic saturation kinetics. The Kin-values for sen Nox of 53 pM and 310 gM are evaluated. Senecionine competitively inhibits sen-Nox uptake, indicating that the tertiary alkaloid and its N-oxide share the same membrane carrier. The N-oxide of sen shows a pH optimum below 5.5, whereas sen is taken up over a range from pH 4 to 8. Activation energies of 90 and 53 kJ.mol-1 are calculated for sen-Nox and sen transport, respectively. At concentrations of 10 to 100 pM, sen-Nox is rapidly taken up by cells and protoplasts; within 2 h > 90% of total N-oxide is within the cells. By contrast the uptake of sen is less efficient. Vacuoles isolated from protoplasts preloaded with sen-Nox totally retained the alkaloid N-oxide, whereas sen is rapidly lost during the procedure of vacuole preparation. N-oxidation converts the weak lipophilic tertiary base into a charged polar molecule which is excellently adapted to serve as the cellular transport and storage form of pyrrolizidine alkaloids.