External metabolites

We seek to describe the time-dependent behavior of a metabolic network that consists of m metabolic reactants (metabolites) interacting via a set of r biochemical reactions or interconversions. Each metabolite S, is characterized by its concentration 5,(f) > 0, usually measured in moles/volume. We distinguish between internal metabolites, whose concentrations are affected by interconversions and may change as a function of time, and external metabolites, whose concentrations are assumed to be constant. The latter are usually omitted from the m-dimensional time-dependent vector of concentrations S(t) and are treated as additional parameters. If multiple compartments are considered, metabolites that occur in more than one compartments are assigned to different subscripts within each compartment. 

Khailov, K.M., 1964. The formation of organic metallic complexes with the participation of external metabolites of marine algae. Dokl. Adad. Nauk. SSSR. Biol. Sci., 155 237-239. 

Every metabolite inside the system boimdary corresponds to a row in the stoichiometric matrix however, some of these metabolites are intracellular, while the rest are extracellular (see Fig. 4). The stoichiometric matrix is arranged such that the m internal metabolites are entered first, and the me external metabolites second m = rtii + me). When the stoichiometric matrix is arranged in this manner, the Suse and the U matrices take on the following form ... 

This concept of chemical ecology was initially developed from works carried out in the terrestrial field, which lent themselves more easily to experimentation (Alexander, 1971 Pasteels, 1972). In the marine sphere, the hypothesis of an intra- and interspecific control by external metabolites was first postulated forty years ago by Lucas (1938, 1947, 1955, 1961), and then adopted more recently by Nigrelli (1958a, b), Fontaine (1970) and Todd et jd. (1972). The theory was generalised by Aubert (1971) following a number of works ceirried out at the C.E.R.B.O.M., essentially in the field... 

Figure 3.1 Growth- and non-growth-coupled toy metabolic networlc (panels a and b, respectively) along with their associated phenotypic yield spaces (panels c and d, respectively). Both networks consist of nine irreversible reactions (diamond ffl -ff9), four internal metabolites (full rectangles), four external metabolites (checkered rectangles BM. biomass P, product of interest Q by-product 5, substrate note that the metabolite Q is the produa of two reactk>ns-A5 and A9) and five EFMs. In the phenotypic yield space EFMs are represented by full circles. Note that the point (1/0) in the phenotypic yield space of network B represents two EFMs with identical yields. The feasible yield space is bounded by the two axes and the dashed line. Growth-coupled production of P is achievable only in network A, but not in network B.

Model Internal metabolites External metabolites Total reactions Irreversible reactions EFMs... 

Fig. 2.1 Tools for metabolic network analysis, (a) Problem Statement The simple depicted network is comprised of internal metabolites A, B, C, D, E, and P linked through the internal reactions (e.g. r, r, r, r, r, and r and external metabolites (e.g. and P J which are transported...

The simplest method for obtaining flux data is to directly measure the uptake or release of selected metabolites by the cells or tissue systems. For example, it is fairly easy to measure the uptake of glucose by cells in a bioreactor. Once the external metabolite fluxes are obtained, the modeling techniques described in the following sections can be used to estimate the associated internal metabolic fluxes. 

On the other hand, in metabolic systems such as the synthesis of tryptophan, histidine, and many other substances, when the internal or external metabolite (usually the end product of the reaction) inhibits instead of inducing the system (feedback inhibition), it is postulated that this end product combines with a prorepressor, converting it into active repressor. Such inhibition is necessary when the temporary appearance of a utilizable substance... 

While it is easy to add materials to a fermentation, removal is difficult. Membrane devices have been placed in the fermenter or in external recycle loops to dialyze away a soluble component. Cells release wastes or metabolites that can be inhibitory these are sometimes referred to as staling factors. Their removal bv dialysis has allowed cell concentrations to reach ten to one hundred times that of control cultures. 

Because of their strategic localization, astrocytes play a crucial role in maintaining the extracellular ionic homeostasis, provide energetic metabolites to neurons and remove excess of neurotransmitter in schedule with synaptic activity. In addition, the strategic location of astrocytes allows them to carefully monitor and control the level of synaptic activity. Indeed, number of papers during the last 15 years have shown that cultured astrocytes can respond to a variety of neurotransmitters with a variety of different patterns of intracellular calcium increases (Verkhratsky et al. 1998). Later on, studies performed in intact tissue preparations (acute brain slices) further established that the plasma membrane receptors can sense external inputs (such as the spillover of neurotransmitters during intense synaptic activity) and transduce them as intracellular calcium elevations, mostly via release of calcium from internal stores (Dani et al. 1992 Murphy et al. 1993 Porter and McCarthy... 

Besides, it is known that the culture medium acts as a common external sink like a lamella (15) or a vacuole (19), in which polysaccharides, enzymes and other metabolites are secreted during growth. Consequently, the growth of plant cell suspensions is a complex process, connected with structural and metabolite changes both in the cell wall and in the culture medium, involving a complex of hydrolytic enzymes. 

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... 

Pretreatment of hair samples also includes an extraction, usually with an alkaline sodium hydroxide solution, followed by cleaning up with LLE with n-hexane/ethyl acetate. Instead of LLE, the employment of SPE is also possible. Furthermore, the solid phase microextraction (SPME) in combination with head-space analysis is usable [104-106]. In the case of using hair samples, possible external contamination (e.g., by passive smoking of Cannabis) has to be considered as false positive result. False positive results can be avoided by washing of the hair samples previous to extraction [107]. Storage of collected samples is another important fact that can cause false results in their content of A9-THC and metabolites [108-110]. 

---