Further Studies of Sequestration of Alkaloids in Papaver Somniferum L
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Further Studies of Sequestration of Alkaloids in Papaver somniferum L. Latex Vacuoles Margaret F. Roberts, B. C. Homeyer, and Thi D. T. Pham The Department of Pharmacognosy, The School of Pharmacy, London University, 29/39 Brunswick Square, London WC 1 N 1 AX Z. Naturforsch. 46c, 377-388 (1991); received December 12, 1990 Papaver somniferum, Latex Vacuoles, Alkaloid, Sequestration 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 (ApH), controlled dissipation of ApH through the use of NH 4C1, and correlation with morphine up take demonstrated that morphine uptake was strongly dependant on the maintenance of ApH. 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-[Ul4C]malate, important acid constituents of the 900 x g vacuoles was investigated. Uptake of sulphate and malate was continuous and satura tion 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-methoxycanthin- 6-one were taken up, but less effec tively than morphine or noscapine. Uptake of alkaloids by P. somniferum latex vacuoles is dependant on the maintenance of tonoplast ApH and an ATPase generates this ApH. Sequestration appears to involve protona tion and anion-cation stabilization involving meconate and sulphate. The specificity of alka loid uptake suggested no clear correlation with pK or lipophilicity, and some sort of channel mechanism, more related to alkaloid shape is suggested. Introduction cytoplasmic environment, for the latex this is nor In higher plants the cell vacuole has been shown mally at pH 6.2-6.8 [3]. These vacuoles also accu in many instances to be the storage compartment mulate low levels of dopamine [11] and have been for alkaloids [1-6]. Previous work on alkaloid shown to contain most of the meconic acid found in the latex [12]. storage in Papaver somniferum has shown that alkaloids are exclusively stored in vacuoles con Two distinct mechanisms for alkaloid transport tained within the latex [7, 8] and in this respect into vacuoles have been proposed; 1. The lipophil alkaloid sequestration in Papaver somniferum is ic alkaloids can penetrate membranes by diffusion similar to that found in Chelidonium majus [9, 10]. and are trapped in the acidic vacuolar compart ment by protonation and salt formation [2, 4, Like other plant vacuoles, P. somniferum latex vac uoles have an internal pH lower than that of their 13-15]. This has been referred to as the ion trap mechanism. 2. Alkaloid uptake across the vac Abbreviations: HEPES, N-2-hydroxyethyl piperazine- uolar tonoplast may occur via catalyzed transport N-2-ethane sulphonic acid; FCCP, carbonylcyanide and some highly specific tonoplast transport sys 4-(trifluoromethoxy)phenylhydrazone; DCCD, N,N'-di- cyclohexyl carbodiimide; MgATP, magnesium + adeno- tems have been reported in a variety of plants [5, 6, syltriphosphate; ApH, transmembrane proton gradient. 16]. It has been reported that alkaloid uptake into Reprint requests to Dr. M. F. Roberts. vacuoles isolated from Fumaria protoplasts could Verlag der Zeitschrift für Naturforschung, D-7400 Tübingen be stimulated by the addition of MgATP indicat 0939-5075/91/0500-0377 $01.30/0 ing that an ATPase activity was necessary for effi- 378 M. F. Roberts et al. ■ Latex Vacuole Sequestration of Alkaloids cient transport. A similar mechanism is required sedimented by centrifuging the collected latex at for lupanine sequestration in vacuoles isolated 900 x g for 30 min and the supernatant of that cen from Lupinus [6] and for senecionine-N-oxide in trifugation at llOOxg for 30 min. The vacuoles Senecio vulgaris [17], The vacuoles of P. somnife were washed twice, and finally suspended to the rum latex may be considered as specialist vacuoles original volume of buffer plus latex in a buffer of due to their size, resistance to external measures 700 m M mannitol, 100 m M HEPES adjusted to designed to reduce the ApH and capacity to pH 6.8 with triethanolamine. This suspension was sequester millimolar amounts of alkaloid. The rap equivalent to 40 to 60 |il (900 x g vacuoles) and 15 id, specific uptake of the major opium alkaloids to 20 |il (1100 x g vacuoles) intravacuolar volume has no absolute requirement for MgATP, does not for each 200 |il assay sample. show saturation kinetics and is largely insensitive to temperature [3], FCCP causes some reduction in alkaloid uptake Determination of vacuolar volume suggesting that a proton gradient across the vac The intravacuolar volume of the 900 x g vacuole uole tonoplast is essential for continued alkaloid sample was determined by incubating a 200 |il ali accumulation [3] which in turn implies the exist quot of isolated and resuspended vacuoles with a ence of a proton pump. The mechanism of seques 30 (il aliquot of tritiated water (1.11 * 103 Bq) and tration of alkaloids in P. somniferum therefore did a 10 (j.1 aliquot of a 14COOH-dextran-carboxyl so not clearly fit either hypothesis 1 or 2. In an at lution (3.7 x 102 Bq) for 30 min at room tempera tempt to resolve some of the conflicting results ture (approximately 24.5 °C). In experiments test previously obtained, further investigations of the ing the effect of substances on the transmembrane mechanism of uptake and sequestration of mor- proton gradient the substances were added during phinan alkaloids by the vacuoles of P. somniferum this incubation. The sample was centrifuged in an latex were undertaken. Eppendorf tube at 900 x g for 15 min and a sample of the supernatant counted for 3H and 14C in a Experimental Packard Tri-Carb Liquid Scintillation Spectrome ter using Aquasol LSC cocktail. The pellet was Chemicals suspended in 500 [il of 0.1 n HC1 in methanol, di Unless otherwise stated, chemicals were pur gested for 30 min at room temperature, centri chased from Sigma, U.K. or The British Drug fuged at 2000 x g for 15 min and the entire super Houses, U.K. [14C]methylamine hydrochloride natant counted for 3H and 14C. Counts were cor (1.48-2.22 GBq m M '1), H235S04 (44.4-51.8 rected for quenching, counting efficiency, and TBq m M " 1) and l4COOH-dextran-carboxyl (18.5 — channel overlap. The concentration of tritiated 74.0 MBq g_l) were purchased from New England water was assumed to be constant throughout the Nuclear, U.S.A.; 3H20 (185 MBq ml-'), l-[U14C]- sample and the dextran restricted to the extra- malic acid (1.5-2.2 GBq m M “ 1), [,4CH3]morphine vacuolar space. Dividing 3H counts per |il of (2.07 GBq m M “ 1) was purchased from Amersham supernatant into the 3H counts from the pellet pro International, U.K.; and ATP was purchased from vided the total sample void volume trapped in the Boehringer, F.R.G. pellet; dividing the 14C counts per [il of superna tant into the 14C counts from the pellet provided Plant material the trapped extravacuolar void volume; subtract Papaver somniferum cv. Halle was cultivated at ing the trapped extravacuolar void volume from The School of Pharmacy Medicinal Plant Garden. the total sample void volume left the intravacuolar Plants were grown in sequential batches so that volume [20]. This volume was very consistent for flowering occurred from late June until end of all similarly aged latex samples in a given season. October. Isolation of latex vacuoles Determination of intravacuolar pH Latex was collected into 700 m M mannitol to a The distribution of the membrane permeable final concentration of 50% latex. Vacuoles were base, methylamine, has been successfully used for M. F. Roberts et al. ■ Latex Vacuole Sequestration of Alkaloids 379 the determination of intracellular and intravac- Isolation of unlabelled alkaloids in uptake uolar pH values [19, 20], The method is based on specificity experiments the assumption that uncharged compounds can 1. Noscapine and nicotine easily pass across a membrane whereas charged forms are impermeant. Since the ratio of charged P. somniferum latex vacuole samples, 900 x g to uncharged compound is governed by the pH and 1100 x g were plasmolyzed in 0.025 n HC1 in value, that value can be deduced. For most thor MeOH. After centrifugation the supernatant from oughly studied situations, these assumptions have each sample was evaporated to dryness and the proved correct [21]. The possibility of the methyl- residue taken up in a small volume of 2% sulfuric amine binding to the membrane must be consid acid. The pH was adjusted to 9.5 with NH4OH the ered carefully; in a vacuolar system, the possibility sample placed on a column of Extrelut (Merck, of differing effects of a transplasmalemma and a Darmstadt) and the alkaloids isolated by elution transtonoplast potential does not occur [22].