Adenine nucleotide translocase-dependent anion transport in pea chloroplasts

Further insight into mechanism of action of clodronate: inhibition of mitochondrial ADP/ATP translocase by a nonhydrolyzable, adenine-containing metabolite

Bisphosphonates are currently the most important class of antiresorptive drugs used for the treatment of diseases with excess bone resorption. Recent studies have shown that bisphosphonates can be divided into two groups with distinct molecular mechanisms of action depending on the nature of the R(2) side chain. Alendronate, like other nitrogen-containing bisphosphonates, inhibits bone resorption and causes apoptosis of osteoclasts and other cells in vitro by preventing post-translational modification of GTP-binding proteins with isoprenoid lipids. Clodronate, a bisphosphonate that lacks a nitrogen, does not inhibit protein isoprenylation but can be metabolized intracellularly to a beta-gamma-methylene (AppCp-type) analog of ATP, which is cytotoxic to macrophages in vitro. The detailed molecular basis for the cytotoxic effects of adenosine-5'-[beta,gamma-dichloromethylene]triphosphate (AppCCl(2)p) has not been determined yet. We addressed this question by studying the effects of alendronate, clodronate, and the clodronate metabolite AppCCl(2)p on isolated mitochondria, mitochondrial fractions, and mitochondrial membrane potential in isolated human osteoclasts. We found that AppCCl(2)p inhibits mitochondrial oxygen consumption by a mechanism that involves competitive inhibition of the ADP/ATP translocase. Alendronate or the native form of clodronate did not have any immediate effect on mitochondria. However, longer treatment with liposome-encapsulated clodronate caused collapse of the mitochondrial membrane potential, although prominent apoptosis was a late event. Hence, inhibition of the ADP/ATP translocase by the metabolite AppCCl(2)p is a likely route by which clodronate causes osteoclast apoptosis and inhibits bone resorption.

A possible role for pyrophosphate in the coordination of cytosolic and plastidial carbon metabolism within the potato tuber

The early stages of tuber development are characterized by cell division, high metabolic activity, and the predominance of invertase as the sucrose (Suc) cleaving activity. However, during the subsequent phase of starch accumulation the cleavage of Suc occurs primarily by the action of Suc synthase. The mechanism that is responsible for this switch in Suc cleaving activities is currently unknown. One striking difference between the invertase and Suc synthase mediated cleavage of Suc is the direct involvement of inorganic pyrophosphate (PPi) in the latter case. There is presently no convincing explanation of how the PPi required to support this process is generated in potato (Solanum tuberosum) tubers. The major site of PPi production in a maturing potato tubers is likely to be the reaction catalyzed by ADP-glucose pyrophosphorylase, the first committed step of starch biosynthesis in amyloplasts. We present data based on the analysis of the PPi levels in various transgenic plants altered in starch and Suc metabolism that support the hypothesis that PPi produced in the plastid is used to support cytosolic Suc breakdown and that PPi is an important coordinator of cytosolic and plastidial metabolism in potato tubers.

Homeostasis of adenylate status during photosynthesis in a fluctuating environment

This review describes and assesses pathways likely to influence and stabilize the ATP/reductant balance during whole cell photosynthesis. The sole reductive step of the Calvin cycle occurs during the conversion of 3-phosphoglycerate to triose phosphate. Photophosphorylation linked to this reaction can undoubtedly supply most of the ATP required by the Calvin cycle and other chloroplastic reactions. Small but crucial contributions must come from several other pathways, some of which involve co-operation between the chloroplast and the rest of the cell. Extrachloroplastic compartments can contribute to chloroplastic ATP requirements by supplying ATP directly or, probably more significantly, by accepting reducing equivalents and so supporting ATP synthesis within the chloroplast.

The contribution of mitochondria to energetic metabolism in photosynthetic cells

Mitochondria fulfill important functions in photosynthetic cells not only in darkness but also in light. Mitochondrial oxidative phosphorylation is probably the main mechanism to supply ATP for extrachloroplastic functions in both conditions. Furthermore, during photosynthesis mitochondrial electron transport is important for regulation of the redox balance in the cell. This makes mitochondrial function an integral part of a flexible metabolic system in the photosynthetic cell. This flexibility is probably very important in order to allow the metabolism to override disturbances caused by the changing environment which plants are adapted to.

Transport of inorganic pyrophosphate across the spinach chloroplast envelope

Spinach-leaf chloroplasts take up PPi at a rate of 1.9 mumol/h per mg of chlorophyll (Chl) in the dark and 1.6 mumol/h per mg of Chl in the light. The Km for PPi transport is 32 microM in the dark and 6 microM in the light. Uptake is inhibited by pyridoxal phosphate, 4,4'-di-isothiocyanatostilbene-2,2'-disulphonic acid and imidodiphosphate, but not by NaF or EDTA. PPi does not appear to cross the chloroplast envelope in exchange for Pi, suggesting that it is not transported by the phosphate translocator. Exchange of PPi and adenine nucleotides across the chloroplast envelope is very slow and PPi does not competitively inhibit ATP uptake, suggesting that little, if any, PPi is transported by the adenine-nucleotide translocator. These results are consistent with the presence of a specific, high-affinity PPi translocator in the spinach chloroplast envelope. It is proposed that in vivo PPi is taken up into the chloroplast from the cytosol to replenish the Pi pool in the stroma.

Evidence for adenylate nucleotide transport (ATP-ADP translocation) in vesicles of Frankia sp. strain EAN1pec

Atractyloside and carboxyatractyloside partially inhibited nitrogenase activity (acetylene reduction) by isolated vesicles of Frankia strain EAN1pec. Extracts of disrupted vesicles showed nitrogenase activity that was not affected by the inhibitors. The vesicles accumulated ATP by an atractyloside-sensitive mechanism. This inhibition of ATP uptake was reversed when vesicles were permeabilized by detergent. Uptake of ATP was inhibited by excess ATP and ADP, but not AMP or adenosine, and by a calcium-dependent ATPase inhibitor. Uptake was stimulated by calcium ions. Accumulation of ATP was accompanied by release of ADP and AMP from the vesicles. The ATP taken up by vesicles and cells grown with N2 as the nitrogen source was found in the corresponding cell pools only as ATP. The data indicate activity of an ATP-ADP translocase system in vesicles of this organism. The role of ATP translocation in the symbiosis between Frankia strain EAN1pec and plant root nodules is discussed.