A General Aspects

In addition to the primary metabolic reactions which are similar in all groups of living organisms (formation and breakdown of nucleic acids and proteins and their precursors, of most carbohydrates, of some carboxylic acids, etc., cf. Table 6), a vast number of metabolic pathways lead to the formation of pecuhar chemical compounds, the so-caUed secondary products . The most characteristic features of these substances are 

Whereas some aspects of secondary metabolism are now relatively well known, e.g., chemistry and biochemistry, others are still obscure. This is especially true for the mode of action of most signals influencing secondary metabolism and the molecular mechanisms which integrate secondary product biosynthesis into the programs of differentiation and development. The latter may be better understood when the molecular organization of these programs is known in more detail (Nover et al. 1982). 

Different criteria have been used for the definition of secondary metabohsm during the development of biochemistry. Kossel (1891) first applied the designation secondary to certain ceU constituents. In a lecture before the Physiological Society of Berlin he stated (cf. Mothes 1980)  

It was, however, not before 1950 that the books of Paech and Bonner brought the term secondary products into broader use. 

However, the distribution of the individual compounds is restricted. This was first pointed out by Sachs (1882) and Pfeffer (1897), who demonstrated the lack of phylogenetic continuity of secondary substances such as oxalate, resins, and essential oils in the plant kingdom. Usually the more chemical reactions necessary for the synthesis of a given secondary product, the more restricted is its distribution. The alkaloid, nicotine, which is formed by a relatively simple biosynthetic pathway (D 16.2) occurs in many different plant species, whereas the much more complicated alkaloid brucine (D 21.3), is synthesized by only one genus of the Loganiaceae. In other words, the probabiUty that a given metabolic pathway 

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III. REACTIVITY OF METALS AND LIGANDS IN VIVO III.A. General Aspects... 

The Stern-Volmer equation (see sect. 4) may be used to determine small amounts of a species which would behave as an inhibitor of a given luminescent probe. The detection limit depends, among other parameters, on and on the detection limits of the setup. The potentials of this method for analytical purposes are discussed, on a general aspect, in Borissevitch (1999), Rakicioglu et al. (1998) and the specific cases of Eu(III) or U(VI) are presented in Georges (1993), Lopez and Birch (1996), Kessler (1998). For example, a detection limit of 7 ngl-1 for Cu2+ is obtained (Lopez and Birch, 1996). Numerous factors may render the method difficult to apply besides the variations of ksv as a function of ionic strength, if more than one quencher is present in solution, it becomes difficult to determine their individual concentrations. This problem has been studied in the case of solutions that more or less mimic the nuclear fuel solutions in Katsumura et al. (1989). 

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