metabolite [85, 97]. This is an important finding, since so far only plasma
membrane-localized MATEs had been found to be involved in detoxification
(reviewed in [98]). It is likely that further studies will reveal a role for vacuolar
MATEs in cellular detoxification. Sequestration of detoxified compounds seems
beneficial for the living plant cell, and the vacuole might be regarded as final storage
compartment. Break down to smaller metabolites [95] or adding a malonyl residue
alters the molecule so that backflush through the ABC transporters is prevented and
final storage in vacuoles occurs [99, 100]. Interestingly, in umbrella papyrus the
oxybenzone conjugate also undergoes partial cleavage and subsequent
malonylation [85].
The significance of such phase III sequestration mechanism for the uptake of
xenobiotics may be understood from the membrane potential across the tonoplast,
which is À30 to À40 mV, and maintained by the activity of ATPases [101]. Since
most ABC transporters are antiporters, the extrusion of cations leads to the accumulation of organic anions by a factor of 3 or 4 [96]. Such an efficient flow of
xenobiotic metabolites will lead to a diminished cytosolic concentration of the active
parent compound and hence be a strong driver for further diffusive uptake into
the cell.
3 Experimental Section
For a bibliographic online search (using the search engine Google Scholar) of the
scientific literature on plant uptake of pharmaceutical compounds, crossing 7 years
of publications, authors used a combination of keywords as “plant uptake + pharmaceutical group” or “plant uptake + compound name” to obtain the highest number
of articles within the topic and pharmaceutical group. Parameters like concentration
applied in the study, time of exposure, type of experiment (hydroponic, pot or plate
experiment), final concentration in the plant or plant part with clear units and plant
species were used to decide which articles would be part of the study.
Field and lysimeter studies were not included due to their complexity and the
number of external factors that can influence the results and therefore may not be
compatible with the other studies. Experiments with different time points where
concentrations in nutrient media/soil were not mentioned for the middle time points
were also excluded, since it was not possible to calculate bioconcentration factors for
these cases. Moreover, when no numerical data was provided in the studies, approximate values were extracted from figures with support of ImageJ software (version
1.52a) using the tools “set scale” and “analyse”.
Chemical properties like molar mass (g/mol), logarithmic octanol-water partition
coefficient (log K OW ) and water solubility (mg/L) were gathered from PubChem
and/or DrugBank website, while the acid dissociation constant (pKa) and the
logarithmic distribution coefficient (log D OW ) were calculated using the software
SPARC Performs Automated Reasoning in Chemistry and values used according to
the pH measured in each article.
Uptake and Translocation of Pharmaceuticals in Plants:. . .
117
membrane-localized MATEs had been found to be involved in detoxification
(reviewed in [98]). It is likely that further studies will reveal a role for vacuolar
MATEs in cellular detoxification. Sequestration of detoxified compounds seems
beneficial for the living plant cell, and the vacuole might be regarded as final storage
compartment. Break down to smaller metabolites [95] or adding a malonyl residue
alters the molecule so that backflush through the ABC transporters is prevented and
final storage in vacuoles occurs [99, 100]. Interestingly, in umbrella papyrus the
oxybenzone conjugate also undergoes partial cleavage and subsequent
malonylation [85].
The significance of such phase III sequestration mechanism for the uptake of
xenobiotics may be understood from the membrane potential across the tonoplast,
which is À30 to À40 mV, and maintained by the activity of ATPases [101]. Since
most ABC transporters are antiporters, the extrusion of cations leads to the accumulation of organic anions by a factor of 3 or 4 [96]. Such an efficient flow of
xenobiotic metabolites will lead to a diminished cytosolic concentration of the active
parent compound and hence be a strong driver for further diffusive uptake into
the cell.
3 Experimental Section
For a bibliographic online search (using the search engine Google Scholar) of the
scientific literature on plant uptake of pharmaceutical compounds, crossing 7 years
of publications, authors used a combination of keywords as “plant uptake + pharmaceutical group” or “plant uptake + compound name” to obtain the highest number
of articles within the topic and pharmaceutical group. Parameters like concentration
applied in the study, time of exposure, type of experiment (hydroponic, pot or plate
experiment), final concentration in the plant or plant part with clear units and plant
species were used to decide which articles would be part of the study.
Field and lysimeter studies were not included due to their complexity and the
number of external factors that can influence the results and therefore may not be
compatible with the other studies. Experiments with different time points where
concentrations in nutrient media/soil were not mentioned for the middle time points
were also excluded, since it was not possible to calculate bioconcentration factors for
these cases. Moreover, when no numerical data was provided in the studies, approximate values were extracted from figures with support of ImageJ software (version
1.52a) using the tools “set scale” and “analyse”.
Chemical properties like molar mass (g/mol), logarithmic octanol-water partition
coefficient (log K OW ) and water solubility (mg/L) were gathered from PubChem
and/or DrugBank website, while the acid dissociation constant (pKa) and the
logarithmic distribution coefficient (log D OW ) were calculated using the software
SPARC Performs Automated Reasoning in Chemistry and values used according to
the pH measured in each article.
Uptake and Translocation of Pharmaceuticals in Plants:. . .
117