Energizing the plasmalemma of marine photosynthetic organisms: the role of primary active transport

Generation of ion electrochemical potential differences by primary active transport can involve energy inputs from light, from exergonic redox reactions and from exergonic ATP hydrolysis. These electrochemical potential differences are important for homoeostasis, for signalling, and for energizing nutrient influx. The three main ions involved are H+, Na+ (efflux) and Cl− (influx). In prokaryotes, fluxes of all three of these ions are energized by ion-pumping rhodopsins, with one archaeal rhodopsin pumping H+into the cells; among eukaryotes there is also an H+ influx rhodopsin in Acetabularia and (probably) H+ efflux in diatoms. Bacteriochlorophyll-based photoreactions export H+ from the cytosol in some anoxygenic photosynthetic bacteria, but chlorophyll-based photoreactions in marine cyanobacteria do not lead to export of H+. Exergonic redox reactions export H+ and Na+ in photosynthetic bacteria, and possibly H+ in eukaryotic algae. P-type H+- and/or Na+-ATPases occur in almost all of the photosynthetic marine organisms examined. P-type H+-efflux ATPases occur in charophycean marine algae and flowering plants whereas P-type Na+-ATPases predominate in other marine green algae and non-green algae, possibly with H+-ATPases in some cases. An F-type Cl−-ATPase is known to occur in Acetabularia. Some assignments, on the basis of genomic evidence, of P-type ATPases to H+ or Na+ as the pumped ion are inconclusive.

Primary active transport of organic anions on bile canalicular membrane in humans

Biliary excretion of several anionic compounds was examined by assessing their ATP-dependent uptake in bile canalicular membrane vesicles (CMV) prepared from six human liver samples. 2,4-Dinitrophenyl- S-glutathione (DNP-SG), leukotriene C4(LTC4), sulfobromophthalein glutathione (BSP-SG), E3040 glucuronide (E-glu), β-estradiol 17-(β-d-glucuronide) (E2–17G), grepafloxacin glucuronide (GPFXG), pravastatin, BQ-123, and methotrexate, which are known to be substrates for the rat canalicular multispecific organic anion transporter, and taurocholic acid (TCA), a substrate for the bile acid transporter, were used as substrates. ATP-dependent and saturable uptake of TCA, DNP-SG, LTC4, E-glu, E2–17G, and GPFXG was observed in all human CMV preparations examined, suggesting that these compounds are excreted in the bile via a primary active transport system in humans. Primary active transport of the other substrates was also seen in some of CMV preparations but was negligible in the others. The ATP-dependent uptake of all the compounds exhibited a large inter-CMV variation, and there was a significant correlation between the uptake of glutathione conjugates (DNP-SG, LTC4, and BSP-SG) and glucuronides (E-glu, E2–17G, and GPFXG). However, there was no significant correlation between TCA and the other organic anions, implying that the transporters for TCA and for organic anions are different also in humans. When the average value for the ATP-dependent uptake by each preparation of human CMVs was compared with that of rat CMVs, the uptake of glutathione conjugates and nonconjugated anions (pravastatin, BQ-123, and methotrexate) in humans was ∼3- to 76-fold lower than that in rats, whereas the uptake of glucuronides was similar in the two species. Thus there is a species difference in the primary active transport of organic anions across the bile canalicular membrane that is less marked for glucuronides.