Micro-metabolite

In many cases micro-metabolites are not detectable except by microbial conversions. For instance Bertau and co-workers contributed to elucidating mechanisms ofxenobiotic metabolism on a molecular level [2, 34, 35], delivering a multitude of novel insights into the micro-metabolism of xenobiotics, i.e. drugs, even pointing towards possible side-effects of drug administration. 

The catabolism of 2 includes 118 reactions many of which are reversible (Fig. 3.5). Since kinetic data on micro-metabolites are difficult to determine experimentally, and in order to obtain an overall view of the xenobiotic metabolism, a stoichiometric model of the full network of degradation pathways of 2 was set up in addition to the network shown in Fig. 3.5. This network was then analyzed by means of elementary flux mode analysis [78]. 

As was shown experimentally, micro-metabolites that are capable of acting as enzyme inhibitors may greatly affect product distribution. A biosimulation model can considerably assist drug approval by identifying physiological effects of drug metabolites too dilute in mammalian systems to be detected analytically. These results also show that the prediction of approximate product distribution requires implementation of a much more complex metabolic network, which is currently being done. 

Corrosion associated with the action of micro-organisms present in the corrosion system. The biological action of organisms which is responsible for the enliancement of corrosion can be, for instance, to produce aggressive metabolites to render the environment corrosive, or they may be able to participate directly in the electrochemical reactions. In many cases microbial corrosion is closely associated with biofouling, which is caused by the activity of organisms that produce deposits on the metal surface. 

It is obvious that rapid metabolite production requires high fluxes of carbon through the metabolic systems responsible for its synthesis. The rate of metabolite production, for a wide range of micro-organisms, has been shown to increase with decrease in... 

In dass 1 (ATP requiring), the rate of metabolite production is limited by the micro-organisms capacity to dissipate energy. 

In this Chapter we shall look at the use of micro-organisms to produce organic adds of commerdal importance. Although all of the examples to be mentioned are relatively simple chemically, they are interesting in that they are metabolically diverse. Some are genuine end products of metabolism, while others are compounds considered to be central metabolites in all living cells. These central metabolites are normally present in relatively small, constant amounts. However, some micro-organisms can be "persuaded" to produce enormous yields of these metabolites. 

Adams, R. F., Schmidt, G. J., and Vandemark, F. L., A micro liquid column chromatography procedure for twelve anticonvulsants and some of their metabolites, /. Chromatogr., 145, 275, 1978. 

Jia, L., Liu, B., Terabe, S., Nishioka, T. (2004). Two-dimensional separation method for analysis of bacillus subtilis metabolites via hyphenation of micro-liquid chromatography and capillary electrophoresis. Anal. Chem. 76, 1419-1428. 

F. De Angelis, R. Ceci, R. Quaresima, S. Reale, A. Di Tullio, Investigation by solid phase micro extraction and gas chromatography/mass spectrometry of secondary metabolites in lichens deposited on stone monuments, Rapid Commun. Mass Spectrom., 17, 526 531 (2003). 

The diol epoxide derivative of benzo(a)pyrene, trans-7,8-dihydroxy-anti-9,10-epoxy-7,8,9,10-tetrahydrobenzo(a)pyrene also known as (+) -73,8a-dihydroxy-9ot,10a-epoxy-7,8,9,10-tetrahydrobenzo(a)pyrene,was the first diol epoxide to be synthesized. Interest in this compound was stimulated by the report by Borgen et al. (8) that a metabolite of benzo(a)pyrene, tentatively identified as the trans-7,8-diol ( 1) became covalently bound to DNA in the presence of rat liver micro-somes. Sims et al. ( ) suggested that the active metabolite was a diol epoxide derivative of unspecified stereo chemistry. 

FIGURE 14.3 Fast analysis of control drugs and metabolites using a 15 cm x 300 fim inner diameter capillary column packed with 3 /tin C18 particles (Micro-Tech Scientific MC-15-C18SS-320-EU) operated at 10 /tL/min gradient flow rate. UV at 278 nm. (Source Drug Enforcement Administration, Southwest Laboratory, Vista, California and S. DiPari.)... 

In environmental analytical applications where analyte concentrations, e.g. surfactants or their metabolites, are quite low, extraction and concentration steps become essential. Solid phase extraction (SPE) with cartridges, disks or SPME fibres (solid phase micro extraction) because of its good variety of SP materials available has become the method of choice for the analysis of surfactants in water samples in combination with FIA as well as LC—MS analysis. SPE followed by sequential selective elution provides far-reaching pre-separations if eluents with different polarities and their mixtures are applied. The compounds under these conditions are separated in the MS spectrometer by their m/z ratios providing an overview of the ionisable compounds contained in a sample. Identification in the sense it has been mentioned before, however, requires the generation of fragments. 

Dihaloelimination is a two-electron transfer reaction. Thompson et al. [377] reported reductive dichloroelimination of 1,1,2-TCA and TeCA by hepatic micro-somes from rat Ever, with VC and both tDCE and cDCE as metabolites. Reductive dichloroelimination from hexa- and pentachloroethane by microsomal cytochrome P450 was studied by Nastainczyk et al. [378]. The main products of the in vitro metabolism of hexa- and pentachloroethane were PCE (99.5%) and TCE (96%), respectively, with minor amounts of pentachloroethane (0.5%) and TeCA (4%), respectively, via reductive dechlorination. 

Although demethylation, which occurs in the liver, is normally considered to be a catabolic process, it may result in conversion of an inactive form of a drug to the active form. Thus 6-(methylthio)purine (XXXIX) is demethylated by the rat to 6-mercaptopurine [205]. This demethylation occurs in the liver micro-somes and is an oxidative process which converts the methyl group to formaldehyde [204, 207]. The 1-methyl derivative of 4-aminopyrazolo[3,4-d] pyrimidine (XLI) is demethylated slowly, but 6-mercapto-9-methylpurine (XLII) not at all [208]. The A -demethylation of puromycin (XLlIl) [209, 210], its aminonucleoside (XLIV) [211], and a number of related compounds, including V-methyladenine and V,V-dimethyladenine, occurs in the liver microsomes of rodents [212]. In the guinea-pig the rate-limiting step in the metabolism of the aminonucleoside appears to be the demethylation of the monomethyl compound, which is the major urinary metabolite [213]. The relationship of lipid solubility to microsomal metabolism [214], and the induction of these demethylases in rats by pre-treatment with various drugs have been studied [215]. 

Vastly reduced solvent consumption for micro-separation techniques has advantages in that it gives superior solvent suppression when protonated solvents are used [88]. Reduced solvent volumes also make the use of fully deuterated solvents more attractive, eliminating the need for solvent suppression [87]. A low-volume capillary probe with a 7 pi cell volume (1.5 pi active) is commercially available and its application to metabolite identification has been reported [89]. 

Toxicological studies have suggested that the species specificity for induction of ovarian tumors (produced in mice but not rats) occurs because the blood level of the ovotoxic VCH metabolite VCH-1,2-epoxide is dramatically higher in VCH-treated female mice compared with rats. VCH has been shown to be metabolized by the liver of mice to the ovotoxic metabolite (VCH-1,2-epoxide), which circulates in blood and is delivered to the ovary, where it destroys small oocytes. This destruction of small oocytes is considered to be an early event in carcinogenesis. Species difference in epoxidation of VCH by hepatic micro-somes correlates well with the differences observed in the blood concentration of VCH-1,2-epoxide and VCH ovarian toxicity. Further in vitro studies have found that the rate of VCH epoxidation in humans by human hepatic microsomes was 13- and 2-fold lower than epoxidation by mouse and rat systems respec-tively. Therefore, if the rate of hepatic VCH epoxidation is the main factor that determines the ovotoxicity of VCH, rats may be a more appropriate animal model for humans. 

Plant residues can provide substrates for the production of phytotoxic metabolites by soil microorganisms but they can also support the growth of pathogens and other deleterious micro-organisms. This is illustrated by reference to the problems of establishing crops drilled in the presence of straw residues and of decaying weed and grass residues that have been previously killed with herbicides. 

Another factor rarely considered is the zone of the root system subjected to the microbial metabolite. It is likely that the metabolites will only be formed in particular regions of the soil where there are suitable substrates for producer micro-organisms and it is unlikely that the entire root system will come under the influence of a metabolite. For example, acetic acid is a coinnon microbial fermentation product of cellulose and is phytotoxic ( 5,... 

Table II. Effect of Micro-organisnvs cind Bieir Metabolites Formed During 14 Days Deconposition of Plant Residues on Longest Root Length (mm) of Barley Seedlings...

---