Biological factors affecting metabolism

Factors affecting metabolism and disposition of drugs or xenobiotics in vivo have been amply described elsewhere (Timbrell, 1991). They may be divided into biological factors and molecular factors. 

TABLE 32.7 Biological Factors Affecting Xenobiotic Metabolism. ... 

This chapter is about both. Our first objective in writing it was obviously to supply as much useful information as would fit in the allocated pages. By useful information, mean structured data as exemplified by the classification of metabolic reactions (Sections 2 and 3) and biological factors affecting them (Section 4). 

A biologically important factor affecting drug absorption is drug metabolism or reaction coincident with diffusion across a membrane. The reaction often produces inactive or less potent products than the parent drug. It is conceivable that the reaction will also reduce the drug flux into the systemic circulation. We are interested in the effect of reaction on membrane diffusion. 

There are many factors, both chemical and biological, which affect the disposition of xenobiotics. Chemical factors include size and structure, pKa, chirality, and lipophilicity. Biological factors include species, sex and strain, genetic factors, hormonal influences, disease and pathological conditions, age, stress, diet, dose, enzyme induction and inhibition, and tissue and organ specificity. All of these factors can affect the toxicity of a chemical by changing its disposition, especially its metabolism. 

Biological factors. The species of animal is a very important factor especially affecting metabolism but also influencing the other phases of disposition. The pH of the gastrointestinal tract, the nature of the skin, and breathing rate all may affect absorption. Plasma proteins and fat can affect distribution. Biliary excretion is affected by species in relation to the molecular weight threshold. 

In solution, the anthocyanins actually might exist in equilibrium with essentially four molecular forms - the flavylium cation, the quinoidal base, the hemiacetal base and chalcone [19]. The relative amounts of the four structure forms depends on both the pH and the difference in structure of the anthocyanins [20-22], Generally, anthocyanins exist primarily as the stable flavylium cation above pH 2. This uniqueness in the chemical structure is one of the important key factors affecting their absorption, metabolism, bioavailability and, consequently, the biological responses of the human body to anthocyanins. 

The first chapter deals with metabolic factors affecting the biological activities of androgens. In the second chapter the structure-activity theories are reviewed. A new approach to the structure-activity relationship is offered in Chapter 3. The effects of structural and stereochemical changes on biological activities are extensively analyzed in this chapter. A new theory of steroid-receptor interaction concludes this chapter. 

Detrital processing can be thought of as a continuum from fresh litter to stabilized SOM (Agren and Bosatta, 2002). At different stages in this continuum, the relative importance of each of these environmental and biological factors that have been identified as controlhng decomposition dynamics will likely vary. The initial stages of mass loss are characteristically most affected by climate, resource quality, and, when abundant, soil macrofauna. The physical soil environment also needs to be considered as an important control on the turnover of more humified SOM in the mineral horizons. It is also evident from this literature review that observed correlations between decay rates and decomposition factors are often attributable to both the direct effects of that factor on microbial metabolism and to the indirect interactions with other factors. 

The major benefit of using an in vivo test is that it provides an intact biological system including exposure route-specific absorption, distribution, metabolism, and excretion responses, as well as tissue-specific responses to the parent compound and any metabolite(s) that reach the various tissues. Many of the factors affecting these processes in chemical... 

Table 13.2.2 Factors affecting strobilurin properties Intrinsic activity, physicochemical data, metabolic degradation features, biokinetic properties and biological use patterns. 

Indeed the most important factor affecting postharvest life is temperature. This is because temperature has a profound effect on the rates of biological reactions, specifically metabolism and respiration. Over the physiological range of most crops, 0 to 30 °C increased temperatures cause an exponential rise in respiration. 

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