Metal ions extracellular

Sastry etal. (2003) observed that the extremophilicactinomycete, Thermomonospora sp. when exposed to gold ions reduced the metal ions extracellularly, yielding gold... 

A rather more specific mechanism of microbial immobilization of metal ions is represented by the accumulation of uranium as an extracellular precipitate of hydrogen uranyl phosphate by a Citrobacter species (83). Staggering amounts of uranium can be precipitated more than 900% of the bacterial dry weight Recent work has shown that even elements that do not readily form insoluble phosphates, such as nickel and neptunium, may be incorporated into the uranyl phosphate crystallites (84). The precipitation is driven by the production of phosphate ions at the cell surface by an external phosphatase. 

Integrins constitute a large family of a (3 heterodimeric cell surface, transmembrane proteins that interact with a large number of extracellular matrix components through a metal ion-dependent interaction. The term integrin reflects their function in integrating cell adhesion and migration with the cystoskeleton. 

Selectivity of ion channels for metal ions is of great current interest (66), as it relates to conduction of nerve signals, maintenance of the appropriate metal ion distribution in the intracellular and extracellular... 

Since in mammalian species metals first need to be assimilated from dietary sources in the intestinal tract and subsequently transported to the cells of the different organs of the body through the bloodstream, we will restrict ourselves in this section to the transport of metal ions across the enterocytes of the upper part of the small intestine (essentially the duodenum), where essentially all of the uptake of dietary constituents, whether they be metal ions, carbohydrates, fats, amino acids, vitamins, etc., takes place. We will then briefly review the mechanisms by which metal ions are transported across the plasma membrane of mammalian cells and enter the cytoplasm, as we did for bacteria, fungi and plants. The specific molecules involved in extracellular metal ion transport in the circulation will be dealt with in Chapter 8. 

EDTA penetrates cell membranes relatively poorly and therefore chelates extracellular metal ions much more effectively than intracellular ions. 

Pharmacological and toxicological studies have shown that demetallation can occur in vivo with deposition of free metal ions in bone and liver especially in situations where the chelate has a long residence time in the body. This is the case with non-extracellular chelates like organ-specific contrast agents or with intravascular compounds such as polymers. 

All currently available extracellular contrast agents are gadolinium chelates. This rare earth metal ion exhibits the strongest effect of all elements on the longitudinal relaxation time Tl. The ligands of these chelates belong to two different types of structure Acyclic (open-chain) compounds and macrocyclics. 

To characterize the kinetic stabilities of complexes, the rate constants should be used, determined for the exchange reactions occurring between the complexes and endogenous metal ions (e.g. Cu2+ and Zn2+). Similarly to the equilibrium plasma models, the development of a kinetic model is needed for a better understanding of the relation between the extent of in vivo dissociation and the parameters characterizing the rates of dissociation, the rates of distribution in the extracellular space and the rates of excretion of the Gd3+ complexes. 

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