Cations uptake mechanisms

It is significant that oat plants, which are known to contain the Fe3+ complexor, 2,4-dihydroxy-7-methoxy-l,4-benzoxazin-3-one (128), do not show any significant accumulation of plutonium, or the other actinides. It is possible that this complexing agent is located within plant cells which do not come into contact with the cation transporting mechanisms. Although there is evidence of the existence of microbial hy-droxamates in soil and that hydroxamates do become concentrated in plants (129), there has been no evidence presented yet that hydroxamates are the agents responsible for plutonium uptake into plants. On the other hand there is evidence that EDTA and DTPA can stimulate actinide concentration in plants (See Table 6). 

It is clear from this brief survey that biologically available chemical species can include free cations, anionic and cationic complexes, and neutral molecules, depending on the element concerned. Such a disparate collection implies a range of different uptake mechanisms probably a minimum of three for positive, neutral, and negative charge types. Furthermore, the estimation of biological availability in terms of chemistry becomes more difficult, since not only will different techniques be required for different elements, but some elements are already included in more than one species type in Tables 2 and 3 (e.g. Cd, Hg, Cu). 

The hydrated thallous ion is similar in size to the hydrated potassium ion, and early literature reported that the uptake of T1 cations in muscle cells made use of the specific uptake mechanism developed for potassium. However, later studies, taking account of the complexity of potassium transport, and the different types of potassium channels, have found some differences between the cellular T1 uptake and the potassium uptake. Thus, digoxin that inhibits the Na/K ATP-ase enzyme system as well as the potassium ion-transport, did not affect the ° T1 transport. 

Gentamicin is more toxic to LLC-PKj monolayers when exposed at the apical side, indicating a preferential uptake from the luminal membrane [136]. The uptake mechanism is proposed to be via megalin mediated endocytosis, a protein which is abundantly expressed in the proximal tubule [137]. A pathway delineated in LLC-PKl cells is proposed, whereby internalized aminoglycosides and other small molecular weight cationic compounds are transported from the early and late endosomes, through the Golgi complex. 

Avery, S. V. (1995). Cesium accumulation by microorganisms uptake mechanisms, cation competition, compartmentalization and toxicity. J. Ind. Microbiol. 14, 76-84. 

In this way, a proton gradient arises which drives the ion uptake into the cell by means of the following mechanism. The ion to be taken up is loaded outside with H and is thereby drawn inwardly by the OH . In the case of anions, this is only possible if more protons are transported than negative charges. The cation uptake does not suffer from such problems, but there is sometimes a symport together with H" from outside to inside. So-called transporters assist in this process. These are incorpo-... 

The antiendotoxin activity of cationic peptides is also related to the above uptake mechanism. Endotoxin is in fact LPS, or more precisely the lipid A portion of LPS. As mentioned above, cationic peptides bind to polyanionic LPS (70.128,129). The binding is of high affinity and cooperative (70). This binding can neutralize the ability of LPS to induce TNF in macrophage cell lines or in a murine model, and it reduces endotoxin mortality in galactosamine-sei isitized mice (130,131). 

The LundegSrdh hypothesis required that 4 monovalent anions and not more than 4 be absorbed per uptake of one O2 molecule in anion respiration and that all anions stimulate respiration to the same extent per anion charge absorbed. Experiment showed that neither condition was fulfilled. It also became clear that one of the ions whose uptake most enhanced respiration was the cation NH4 and that respiration was enhanced when ion uptake is restricted to cation uptake as occurs when a root is surrounded by a moist anion exchange resin. This hypothesis also preeludes an active uptake of ions occurring under anaerobic conditions following upon a period of active aerobic respiration it is a mechanism which would not provide for any storage of the ability to promote aetive ion uptake. In this it is contrary to a number of experimental observations. Further, only one carrier is postulated for all anions and it therefore fails to explain how there is no competition in uptake, for instance, between halide ions and sulphate ions, nor between sulphate and nitrate. It equally leaves unexplained competition between cations. 

Mechanisms (1) Alteration of biocide (enzymatic inactivation) (2) Impaired uptake (3) Efflux Chromosomally mediated, but not usually relevant Applies to several biocides Not known Plasmid/Tn-mediated e.g. mercurials Less important Cationic biocides and antibiotic-resistant staphylococci... 

Especially in dicotyledonous plant species such as tomato, chickpea, and white lupin (82,111), with a high cation/anion uptake ratio, PEPC-mediated biosynthesis of carboxylates may also be linked to excessive net uptake of cations due to inhibition of uptake and assimilation of nitrate under P-deficient conditions (Fig. 5) (17,111,115). Excess uptake of cations is balanced by enhanced net re-lea,se of protons (82,111,116), provided by increased bio.synthesis of organic acids via PEPC as a constituent of the intracellular pH-stat mechanism (117). In these plants, P deficiency-mediated proton extrusion leads to rhizosphere acidification, which can contribute to the. solubilization of acid soluble Ca phosphates in calcareous soils (Fig. 5) (34,118,119). In some species (e.g., chickpea, white lupin, oil-seed rape, buckwheat), the enhanced net release of protons is associated with increased exudation of carboxylates, whereas in tomato, carboxylate exudation was negligible despite intense proton extrusion (82,120). 

In experiments with lowland rice Oiyzci saliva L.) it was found that roots quickly exhausted available sources of P and sub.sequently exploited the acid-soluble pool with small amounts deriving from the alkaline soluble pool (18). More recalcitrant forms of P were not utilized. The zone of net P depletion was 4-6 mm wide and showed accumulation in some P pools giving rather complex concentration profiles in the rhizosphere. Several mechanisms for P solubilization could be invoked in a conceptual model to describe this behavior. However using a mathematical model with independently measured parameters (19), it was shown that it could be accounted for solely by root-induced acidification. The acidification resulted from H" produced during the oxidation of Fe by Oi released from roots into the anaerobic rhizosphere as well as from cation/anion imbalances in ion uptake (18). Rice was shown to depend on root-induced acidification for more than 80% of its P uptake. 

Metal toxicity is also affected by physiochemical factors, such as pH and the concentration of divalent cations. Adding divalent cations, such as zinc, has been reported to mitigate toxicity produced by other metals. For example, the addition of 60 pM zinc reduced toxicity in Pseudomonas putida caused by 3 mM cadmium.148 Zinc had no effect on cells grown in the absence of cadmium. Little is understood surrounding the mechanism of protection however, cadmium uptake was observed to be dependent on zinc concentration.149 Zinc was found to be a competitive inhibitor of cadmium uptake. 

The mechanism of uptake and retention of the mono-cationic 99mTc complexes in the myocardium - or other tissues - has not been fully resolved. Most of the mechanistic studies have been conducted with 99mTc (MIBI)6f and "mTc-diphosphine complexes. It has been shown that these mono-cationic complexes are not taken up in myocytes via the Na+/K+-ATPase pump as is 201T1+ [41]. Instead, these cationic tracers are localized and retained in cellular membranes, including mitochondrial membranes [41]. 

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