Further Studies Of Sequestration Of Alkaloids In Papaver Somniferum L. Latex Vacuoles

Abstract

The uptake and sequestration of alkaloids by latex vacuoles of P. somniferum sedimenting at 900 x g and 1100 x g were studied; both populations take up morphine effectively. Morphine uptake by the 1100 x g vacuoles was stimulated by exogenous ATP and, after uptake of large amounts of alkaloid, both populations were stimulated by exogenous ATP to take up further morphine; this effect was no longer present in 900 x g vacuoles isolated from the latex of more mature capsules. Uptake by 1100 x g vacuoles was more sensitive to temperature than that of 900 x g vacuoles. Determination of the proton gradient across the vacuolar membrane (△pH), controlled dissipation of △pH through the use of NH4Cl, and correlation with morphine uptake demonstrated that morphine uptake was strongly dependant on the maintenance of △pH. Inhibitors designed to produce loss of protons from the vacuole had, however, little effect on the system. Nitrate, as an inhibitor of tonoplast ATPase, had an inhibitory effect on morphine uptake by the 1100 x g vacuoles. Uptake of meconate, [35S]sulphate and L-[U14C]malate, important acid constituents of the 900 x g vacuoles was investigated. Uptake of sulphate and malate was continuous and saturation was not reached; the rate of uptake of sulphate and malate was much lower than that of morphine and other alkaloids, and uptake of meconate by these vacuoles was not detected. Studies on specificity of alkaloid uptake with the 900 x g vacuoles indicated no absolute preference for either the ( + )- or the (-)-isomer of codeine. Noscapine uptake was stimulated by ATP under conditions where ATP had no effect on morphine uptake. Nicotine was not taken up, but low levels of caffeine and l-m ethoxycanthin-6-one were taken up, but less effectively than morphine or noscapine. Uptake of alkaloids by P. somniferum latex vacuoles is dependant on the maintenance of tonoplast △pH and an ATPase generates this △pH. Sequestration appears to involve protonation and anion-cation stabilization involving meconate and sulphate. The specificity of alkaloid uptake suggested no clear correlation with pK or lipophilicity, and some sort of channel mechanism, more related to alkaloid shape is suggested