Yield-dose relationships

A more reliable means of providing a reference of -OH in a biological system maybe by means of irradiation with ionizing radiation (von Sonntag et al. 2000). The action of ionizing radiation on an aqueous medium gives rise to OH whose yield/dose relationship (G value) is known (Chap. 2). Apart from this, since biological media are concentrated solutions the formation of the indicator product, e.g., a phenol (ArOH), via the direct effect [expressions (69) and (70)] must in principle be taken into account as well. It can be shown that with k4i [probe]/ k42 [cellular components] above 10 4 the direct effect contributes less than 10%... 

If the precursor of the product is the major constituent and the product does not react with other radicals, then its yield will be linear. In some instances, a product with high reactivity toward primary radicals is formed, resulting eventually in a steady-state level of that product and in a delayed linear increase in an associated secondary product. If the precursor is a minor constituent, then the yield of the product will increase initially with dose and then level off when the precursor is depleted. The different possible yield-dose relationships and their implications for extrapolating data are discussed in earlier work [5]. 

The dose relationship between the thermal and photochemical (ET) pathways via CT complexes can also be seen from a consideration of oxidative C-C cleavage in benzopinacol donors, which readily form vividly colored CT complexes with various acceptors such as chloranil (CA), DDQ, and NO+ [59]. Thus, benzopinacol/CA solutions are stable (at 23 °C) in the dark. However, irradiation of the CT band leads to a slow bleaching of the color with concomitant formation of the retropinacol product. With a stronger acceptor, such as DDQ (E°red = 0.6 V vs. SCE), the deavage of the benzopinacols can also be achieved by thermal means. For example, the CT complex formed upon mixing tetraanisylpinacol and DDQ bleaches within minutes (in the dark) to afford the retropinacol products in quantitative yield [59]. 

The relationship between equiactive agonist concentrations in the absence and presence of antagonist to yield a dose ratio of 2 ([B] = 10-pAi) is then calculated by equating... 

Exposure. Measurement of total phenol in the urine is the most useful biomarker following inhalation exposure to phenol (ACGIH 1991). The test is nonspecific and should not be used when workers are exposed to benzene, to household products, or to medications containing phenol. Dermal exposure may also result in overestimation of inhalation exposure. In persons not exposed to phenol or benzene, the total phenol concentration in the urine does not exceed 20 mg/L and is usually <10 mg/L (ACGIH 1991). Phenol can also be measured in the urine after oral exposure, although a dose-response relationship between oral exposure to phenol and phenol in the urine has not been established. Benzene metabolism yields not only phenol, 1,4-dihydroxybenzene, and their sulfates and glucuronides, but also the benzene-... 

The in vivo metabolism of a homologous series of alkyl carbamates (7.2, Fig. 7.3) has yielded some informative results [13]. The hydrolysis of these esters liberates carbamic acid (7.3, Fig. 7.3), which breaks down spontaneously to C02 and NH3, allowing the extent of hydrolysis to be determined conveniently and specifically by monitoring C02 production. When such substrates were administered to rats, there was an inverse relationship between side-chain hydroxylation and ester-bond hydrolysis. Thus, for compounds 12 the contribution of hydrolysis to total metabolism (90 - 95% of dose) decreased in the series R=Et (ca. 85-90%), Bu (ca. 60-65%), hexyl (ca. 45 - 50%), and octyl (ca. 30%). Ethyl carbamate (urethane) is of particular toxicological interest, being a well-established carcinogen in experimental animals. In vitro studies of adduct formation have confirmed the competition between oxidative toxification mediated by CYP2E1 and hydrolytic detoxification mediated by carboxylesterases [14]. 

Ingestion of yage in healthy volunteers yields plasma concentrations of 10 to 250 ng/mL for harmine and 1.0 to 25.0 ng/mL of harmaline (Callaway et al. 1996). The dose-concentration relationships are linear in this range. DMT shows linear dose-concentration relationships for plasma concentrations between 5 and 1000 ng/mL. Systemically administered j8-carbolines penetrate brain tissue, with relatively even distribution (Moncrieff 1989). DMT taken alone is not absorbed well orally. It may be taken as a snuff or smoked, or mixed with other plants to improve absorption. 

Heagle and associates found a reduction in yield of sweet com and soybean after exposure to ozone at 0.10 ppm for 6 h/day over much of the growing season. These exposures were carried out in field chambers set over soybean plots in the field. They suggested that a threshold for measurable effects on these crops would lie between ozone (oxidant) concentrations of 0.05 and 0.10 ppm for 6 h/day. These values are realistic in terms of growing-season averages in the eastern United States. More of these studies could help to clarify dose-response relationships for economically important crops. Table 11-5 summarizes these long-term, chronic studies. 

In order to evaluate whether a higher dose is better, a dose-response relationship must be established between the total dose of radiation and locoregional control. The dose response relationship was establishedby Choi andCarey (29). Doses of30-35 Gy yielded a local control rate of 20-25 %. Doses of 45-5 0 Gy resulted in a locoregional control rate of 60-70%. Beyond 50 Gy the level of local control is unclear. A Canadian group per-... 

Process parameters involve line speed if dose rate and line speed are combined, the dose delivered to the product to be cured can be calculated. A processor specific yield factor depends on the relationship between the beam current... 

The experimental kinetics of accumulation of the Frenkel defects - F centres in alkali-halide crystals at liquid-helium temperatures - was studied in [17] and [40] within the framework of a model that yields a logarithmic dependence of the concentration of defects on the irradiation dose - equation (6) of Table 7.6). Although we criticized this relationship above, at low radiation doses it can be represented as a polynomial in powers of uqVq resembling equations (3) to (5). At the same time cogent arguments exist favoring the... 

FIGURE 3-4 Relationship between dosing interval and plasma concentrations of the antiasthmatic drug theophylline. A constant intravenous infusion [shown by the smoothly rising line] yields a desired plasma level of 10 mg/L. The same average plasma concentration is achieved when a dose of 224 mg is taken every 8 hours, or a dose of 672 mg every 24 hours. However, note the fluctuations in plasma concentration seen when doses are taken at specific hourly intervals. 

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