Metabolite uptake

Despite the lack of visible necrosis of young and old leaves, it is possible to demonstrate alterations in cell permeability by following metabolite uptake after ozone exposure. If Acala SJ-1 cotton leaves are exposed to ozone and allowed to take up sucrose 2U hr later, there is nearly a doubling of the rate of sucrose uptake by leaves which are maximally susceptible to ozone. 

Many of the metabolite uptake studies cited above rely on combined uptake and incorporation into starch. In order to separate uptake from incorporation, Schott et al.226 extracted amyloplast membrane proteins from potato tubers and reconstituted them into liposomes. These reconstituted liposomes transported Pi, triose phosphates and G6P in a counter-exchange mode. The liposomes were ineffective in the transfer of G1P uptake of ADP-Glc was not tested. Mohlmann et al.236 have used a proteoliposomic system to reconstitute plastid envelope proteins. In this system, ADP-Glc is transported in exchange for AMP. Thus the more widely studied plastid ATP/ADP transporter was not responsible for ADP-Glc uptake. More recently, Bowsher et al.237 reported that wheat endosperm amyloplasts membrane proteins reconstituted into proteoliposomes took up ADP-Glc in exchange for AMP and ADP. In addition, they showed that under conditions of ADP-Glc dependent starch biosynthesis, the efflux of ADP from intact amyloplasts was equal to that of ADP-Glc utilization by starch synthesis. The amyloplast membrane ADP-Glc/ADP transporter was a 38 000 molecular weight integral membrane protein.237... 

Hie most widely used of these is Af-ethylmaleimide (R = CjHj in Fig. 4), which can be very selective for certain SH-groups of an enzyme. It is as a general SH-reagent for detection of SH-functions in purified proteins and in biological processes, such as hormone release, metabolite uptake, membrane transport, etc. 

FIGURE 5.13 Two basic types of biological transport are (a) transport within or between different cells or tissues and (b) transport into or out of cells. Proteins function in both of these phenomena. For example, the protein hemoglobin transports oxygen from the lungs to actively respiring tissues. Transport proteins of the other type are localized in cellular membranes, where they function in the uptake of specific nutrients, such as glucose (shown here) and amino acids, or the export of metabolites and waste products. 

The drug is metabolized rapidly in the liver, kidney, intestinal mucosa, and even red blood cells. Therefore it has a plasma half-life of only 10 min after bolus intravenous application. The major metabolite, uracil arabinoside (ara-U), can be detected in the blood shortly after cytarabine administration. About 80% of the dose is excreted in the urine within 24 h, with less than 10% appearing as cytarabine the remainder is ara-U. After continuous infusion, cytarabine levels in the liquor (cerebro-spinal fluid) approach 40% of that in plasma. Continuous infusion schedules allow maximal efficiency, with uptake peaks of 5-7 pM. It can be administered intrathecally as an alternative to methotrexate. 

Cellular defense mechanisms against toxins (A multistep mechanism for elimination of toxic metabolites and xenobiotics. It involves various transport, oxidation, and conjugation steps.) are usually divided into several steps as it is visualized on Fig. 3. Organic anion transporting proteins (OATPs) are responsible for the cellular uptake of endogenous compounds and... 

Other metabolites of inositol phospholipids, e.g., inositol (1,3,4,5)-tetrakisphos-phate (IP4) may have additional signal transduction roles, particularly in accelerating the uptake of extracellular Ca ion into the SR following a contraction. 

For convenience, the processes identified in Figure 2.1 can be separated into two distinct categories toxicokinetics and toxicodynamics. Toxicokinetics covers uptake, distribution, metabolism, and excretion processes that determine how much of the toxic form of the chemical (parent compound or active metabolite) will reach the site of action. Toxicodynamics is concerned with the interaction with the sites of action, leading to the expression of toxic effects. The interplay of the processes of toxicokinetics and toxicodynamics determines toxicity. The more the toxic form of the chemical that reaches the site of action, and the greater the sensitivity of the site of action to the chemical, the more toxic it will be. In the following text, toxicokinetics and toxicodynamics will be dealt with separately. 

An environmental protocol has been developed to assess the significance of newly discovered hazardous substances that might enter soil, water, and the food chain. Using established laboratory procedures and C-labeled 2,3,7,8-tetra-chlorodibenzo-p-dioxin (TCDD), gas chromatography, and mass spectrometry, we determined mobility of TCDD by soil TLC in five soils, rate and amount of plant uptake in oats and soybeans, photodecomposition rate and nature of the products, persistence in two soils at 1,10, and 100 ppm, and metabolism rate in soils. We found that TCDD is immobile in soils, not readily taken up by plants, subject to photodecomposition, persistent in soils, and slowly degraded in soils to polar metabolites. Subsequent studies revealed that the environmental contamination by TCDD is extremely small and not detectable in biological samples. 

PBPK models have also been used to explain the rate of excretion of inhaled trichloroethylene and its major metabolites (Bogen 1988 Fisher et al. 1989, 1990, 1991 Ikeda et al. 1972 Ramsey and Anderson 1984 Sato et al. 1977). One model was based on the results of trichloroethylene inhalation studies using volunteers who inhaled 100 ppm trichloroethylene for 4 horns (Sato et al. 1977). The model used first-order kinetics to describe the major metabolic pathways for trichloroethylene in vessel-rich tissues (brain, liver, kidney), low perfused muscle tissue, and poorly perfused fat tissue and assumed that the compartments were at equilibrium. A value of 104 L/hour for whole-body metabolic clearance of trichloroethylene was predicted. Another PBPK model was developed to fit human metabolism data to urinary metabolites measured in chronically exposed workers (Bogen 1988). This model assumed that pulmonary uptake is continuous, so that the alveolar concentration is in equilibrium with that in the blood and all tissue compartments, and was an expansion of a model developed to predict the behavior of styrene (another volatile organic compound) in four tissue groups (Ramsey and Andersen 1984). 

Just as the synthesis of DA and NA is similar so is their metabolism. They are both substrates for monoamine oxidase (MAO) and catechol-O-methyl transferase (COMT). In the brain MAO is found in, or attached to, the membrane of the intraneuronal mitochondria. Thus it is only able to deaminate DA which has been taken up into nerve endings and blockade of DA uptake leads to a marked reduction in the level of its deaminated metabolites and in particular DOPAC. The final metabolite, homovanillic... 

Finally, the actions of the so-called 5-HT releasing agent , if-fenfluramine, which is well known for its anorectic effects, should be mentioned here. This compound inhibits 5-HT uptake but its metabolite, if-norfenfluramine, increases 5-HT release as do high doses of (i-amphetamine. It is important to realise that this 5-HT release is independent of nerve impulses and the action of such compounds rests on their effects on the 5-HT transporters on the storage vesicles and terminal membrane. Once these drugs have been taken up into 5-HT neurons by the transporter, they cause 5-HT to leak out of its storage vesicles and, ultimately, to be extruded from the neuron by retrotransport (see below and Chapter 4 for further details). 

The uptake and elimination of benzo[a]pyrene by the terrestrial isopod Porcellio scaber have been investigated (van Brnmmelen and van Straalen 1996), and 1-hydroxypyrene was identified among the metabolites of pyrene in this organism (Stroomberg et al. 1996). 

There has been considerable interest in the use of plants for bioremediation and this merits a rather extensive discussion. Plants can play an important role in bioremediation for several reasons (1) they can transport contaminants from the soil, (2) they can metabolize the contaminants after uptake, or (3) they can produce exudes that support microbial activity for degradation of the contaminants. In addition, bacteria can produce metabolites that counter the effect of toxins produced by fungi, and serve as biocontrol agents that diminish the need for the application of agrochemicals. Plant exudates play an important role in supporting the growth and activity of bacteria that carry out the degradation of contaminants in the rhizosphere and rhizoplane (the external surface of roots... 

In addition to the well-characterized role of iron in catalysing redox interactions, other metallic contaminants, for example, nickel, may also contribute. In vivo toxicity studies have demonstrated the capacity of nickel particulate compounds to induce tumours following intraperitoneal injection (Pott etal., 1987). Such activity is proportional to their phagocytic uptake, and to the associated respiratory burst and generation of PMN-derived reactive oxygen metabolites (ROMs), a proposed pathogenic mechanism (Evans et al., 1992a). 

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