Solubility transport activity

Recently, Prasad et al. cloned a mammalian Na+-dependent multivitamin transporter (SMVT) from rat placenta [305], This transporter is very highly expressed in intestine and transports pantothenate, biotin, and lipoate [305, 306]. Additionally, it has been suggested that there are other specific transport systems for more water-soluble vitamins. Takanaga et al. [307] demonstrated that nicotinic acid is absorbed by two independent active transport mechanisms from small intestine one is a proton cotransporter and the other an anion antiporter. These nicotinic acid related transporters are capable of taking up monocarboxylic acid-like drugs such as valproic acid, salicylic acid, and penicillins [5], Also, more water-soluble transporters were discovered as Huang and Swann [308] reported the possible occurrence of high-affinity riboflavin transporter(s) on the microvillous membrane. 

As has already been pointed out, the solubility properties of the anion, its lipophilicity, are extremely important for the dissolution of the complex in solvents of low polarity. Large and soft inorganic and, much more so, organic anions very strongly increase the solubility. Anion activation and cation transport processes both depend on such anion effects. 

The glutamic acid residue renders this compound soluble in alkaline media stacking to form nanotubes occurs after acidification (Scheme 1). The more hydrophobic side chains in cyclo[-(Trp-D-Leu)3-Gln-D-Leu-] even enabled the construction of a transmembrane ion channel with a proton transport activity similar to that of gramicidin A or amphotericine B 6]. Measurements of singlechannel conductivity showed fast transport of sodium and potassium ions the channels pore diameter of 7.5 A led to weak potassium selec-tiv-ity [7]. 

In addition to nutritional inadequacy, vitamin deficiency may result from malabsorption, effects of pharmacological agents, and abnormalities of vitamin metabolism or utilization. Thus, in biliary obstruction or pancreatic disease, the fat-soluble vitamins are poorly absorbed despite adequate dietary intake because of steatorrhea. Absorption, transport, activation, and utilization of vitamins require the participation of enzymes or other proteins whose synthesis is under genetic control. Dysfunction or absence of one of these proteins can produce a disease that is clinically indistinguishable from one caused by dietary deficiency. In vitamin-dependent or vitamin-responsive... 

In a different approach, a water-soluble, short-chain (dibutyroyl) analog of PC was used to measure the rate of uptake and lumenal sequestration by isolated liver microsomes (W. Bishop, 1985). This PC analog was taken up in a time- and temperature-dependent manner. The kinetics of uptake were saturable with respect to substrate concentration and the transport activity was protease sensitive. The transporter was also shown to be stereospecific in its action and was unaffected by the addition of ATP. Virtually identical properties have also been described for a microsomal transporter that utilizes butyroyl-lyso-PC (Y. Kawashima, 1987). 

The results indicate that the initial rate of transport of PE is rapid and proceeds without a lag (Fig. 8). The transport process is insensitive to metabolic poisons that disrupt vesicle transport and cytoskeletal structure. The rapid transport kinetics occur at rates consistent with a soluble carrier-mediated process or transfer at zones of apposition between membranes. Analysis of the kinetics of the process is complicated since only PE at the outer leaflet of the plasma membrane is measured, and the basal scramblase activity or the leakage of the ATP-dependent aminophospholipid transporter activity within the plasma membrane may be a step required for the lipid to arrive at this location. Despite these complications, the results clearly indicate that the initial rate of arrival of PE at the plasma membrane occurs on a timescale that clearly distinguishes it from well-characterized vesicle transport phenomena, and is independent of processes involved in protein transport to the cell surface. 

A mixed-film phenomenon of particular interest in the biological and medical areas is that referred to as film penetration, in which a soluble surface-active material in the substrate enters into the surface film in sufficient quantity to alter its nature significantly, or to undergo some alternative physical or chemical process related to the surface (Fig. 8.20). Such penetration studies using films of biological materials have been used to mimic phenomena in biological systems (cell walls and membranes, for example) that cannot readily be studied directly. Of particular interest are such topics as cell surface reactions, catalysis, and transport across membranes. 

The rate of photosynthesis was measured using a portable IRGA (ADC, England). Chloroplast isolation and measurements of photochemical activity which included PSII, PSI and whole chain electron transport activities, were carried out according to the procedure described by Sayeed and Mohanty (7). Estimation of chlorophyll was carried out following Arnon (8) and soluble leaf proteins were estimated following Lowry et al (9). 

Surface tension gradients due to differences in the surface excess of soluble surface-active components may exist only when either the surface is not in equilibrium with the bulk composition or there are concomitant differences in bulk composition parallel to the surface. In the case of the former, the magnitudes of the gradients are of course determined by the rate of transport of surfactant to the relevant surfaces. With concentrated surfactant solutions, transport rates by diffusion will be rapid and surface tension gradients will be diminished so that shear rates are only balanced at... 

Using the average river data for Vy> and 1 SD shown in Figure 11.3, obtain estimates of the MTCs for the PCB contaminated bed-sediment of the Grasse River, New York. Based on the computed result, comment on significance of advection on soluble transport from bed to water column. Assume tetrachlorobiphenyl with Defif = 2.20E — 6 cm /s in bed with porosity e = 0.5, and h = 10 cm active transport layer thickness. 

The basic mechanism of passivation is easy to understand. When the metal atoms of a fresh metal surface are oxidised (under a suitable driving force) two alternative processes occur. They may enter the solution phase as solvated metal ions, passing across the electrical double layer, or they may remain on the surface to form a new solid phase, the passivating film. The former case is active corrosion, with metal ions passing freely into solution via adsorbed intermediates. In many real corrosion cases, the metal ions, despite dissolving, are in fact not very soluble, or are not transported away from the vicinity of the surface very quickly, and may consequently still... 

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