Browning chemical manifestations

Of all rat strains, the Brown Norway (BN) rat is used most frequently in relation to chemical-induced autoimmunity. This strain displays clinically manifested autoimmune disease following exposure to a number of chemicals. HgCl2 is the most scrutinized compound in the BN rat but D-penicillamine [6-8], gold-salts [9,10],hexachlorobenzene (HCB) [11-14] and recently, nevirapine [15], have all been shown to induce clinical effects. Captopril [7] and felbamate [16] appeared not to induce autoimmune effects in BN rats. 

The induction period, followed by a sharp increase in rate is, however, the most characteristic feature of autocatalysis in closed vessels. One manifestation of this behaviour is the clock reaction . An experimental system which is a typical chemical clock and which also exhibits cubic autocataiysis is the iodate-arsenite reaction. In the presence of excess iodate, the system which is initially colourless eventually undergoes a sudden colour change to brown (or blue in the presence of starch). The potential of an iodide-sensitive electrode shows a barely perceptible change during most of the induction period, but then rises rapidly, reaching a peak at the point of colour change. 

The Brown Norway rat has long been considered to be a valuable model (Donker et al., 1984 Balazs, 1987), as a number of chemicals have been shown to induce clinically manifested auto-immune-like disease in this particular strain of rats. This model has been used extensively in studies of mercury(II) chloride. D-Penicil-lamine (Tournade et al., 1990), gold salts (Toumade et al., 1991 Qasim et al., 1997), and hexachlorobenzene (Vos et al., 1979a Michielsen et al., 1999a, 1999b Ezendam et al., 2004a, 2004b) have been shown to induce clinical effects in the Brown Norway rat model. Captopril (Donker et al., 1984) and also felbamate (Popovic et al., 2004) were tested in Brown Norway rats as well, but effects were not seen with these exposures. 

Fulvic acids are organic substances which remain in the solution after acid precipitation of humic and hymatomelanic acids from the extract of humic acids. The chemical structure and physico-chemical properties of these substances are similar to those in humic acids. The aromatic character of the molecule is less distinct, and side chains predominate. They also differ from humic acids in their lower molecular weight, yellow to light-brown colour, better solubility in water and easier hydrolysis in acid media. They are easily eluted from soils and peat. They are more acid than humic acids (pH of solution with c = 0.01 mol 1 is about 2.6). They manifest complex-forming properties and exchange capacity. 

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