Sulfate esters are polar and readily excreted in urine. Aging does not affect these processes. From developing new therapies that treat and prevent disease to helping people in need, we are committed to improving health and well-being around the world. The Manual was first published in as a service to the community. Learn more about our commitment to Global Medical Knowledge. Common Health Topics. Videos Figures Images Quizzes. Cytochrome P Test your knowledge. Which of the following methods used by drugs to cross cell membranes is most likely to involve movement against a concentration gradient?
Add to Any Platform. Click here for Patient Education. Was This Page Helpful? Yes No. Drug Excretion. Dexamethasone Phenobarbital Other barbiturates Phenytoin Rifampin. When the prescribed doses are taken, the desired therapeutic response is observed with little or no toxicity. However, when excessive doses of acetaminophen are taken, hepatotoxicity can occur. This is because acetaminophen normally undergoes rapid biotransformation with the metabolites quickly eliminated in the urine and feces.
At high doses, the normal level of enzymes may be depleted and the acetaminophen is available to undergo the reaction by an additional biosynthetic pathway, which produces a reactive metabolite that is toxic to the liver. Biotransforming enzymes, like most other biochemicals, are available in a normal amount and in some situations can be "used up" at a rate that exceeds the body's ability to replenish them. This illustrates the frequently used phrase, the "dose makes the poison. Figure 4.
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Biotransformation reactions are categorized not only by the nature of their reactions, for example, oxidation, but also by the normal sequence with which they tend to react with a xenobiotic. Phase I reactions are generally reactions which modify the chemical by adding a functional structure.
This allows the substance to "fit" into a second, or Phase II enzyme, so that it can become conjugated joined together with another substance. Phase II reactions consist of those enzymatic reactions that conjugate the modified xenobiotic with another substance. The conjugated products are larger molecules than the substrate and generally polar in nature water soluble.
Thus, they can be readily excreted from the body. Conjugated compounds also have poor ability to cross cell membranes.
In some cases, the xenobiotic already has a functional group that can be conjugated and the xenobiotic can be biotransformed by a Phase II reaction without going through a Phase I reaction. For example, phenol can be directly conjugated into a metabolite that can then be excreted. As illustrated in Figure 5, benzene is biotransformed initially to phenol by a Phase I reaction oxidation. Phenol has a structure including a functional hydroxyl group that is then conjugated by a Phase II reaction sulfation to phenyl sulfate.
Figure 5. Biotransformation of benzene into phenol in Phase 1 oxidation , which is then conjugated by a Phase 2 reaction sulfation to phenyl sulfate Image Source: NLM. Table 1 lists the major transformation reactions for xenobiotics broken into Phase I and Phase II reactions. These reactions are discussed in more detailed below. Phase I biotransformation reactions are simple reactions compared to Phase II reactions.
In Phase I reactions, a small polar group containing both positive and negative charges is either exposed on the toxicant or added to the toxicant. The three main Phase I reactions are 1 oxidation; 2 reduction; and 3 hydrolysis.
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Oxidation is a chemical reaction in which a substrate loses electrons. There are a number of reactions that can achieve the removal of electrons from the substrate. The specific oxidizing reactions and oxidizing enzymes are numerous and several textbooks are devoted to this subject. Most of the reactions are described by the name of the reaction or enzyme involved. Some of these oxidizing reactions include:. Reduction is a chemical reaction in which the substrate gains electrons. Reductions are most likely to occur with xenobiotics in which oxygen content is low.
Reductions can occur across nitrogen-nitrogen double bonds azo reduction or on nitro groups NO 2.
Frequently, the resulting amino compounds are oxidized which forms toxic metabolites. Some chemicals such as carbon tetrachloride can be reduced to free radicals, which are quite reactive with biological tissues. Thus, reduction reactions frequently result in activation of a xenobiotic rather than detoxification. An example of a reduction reaction in which the nitro group is reduced is illustrated in Figure 7. Figure 7. Reduction reaction in which the nitro group is reduced from NO 2 to NH 2. There are fewer specific reduction reactions than oxidizing reactions. The nature of these reactions is also described by their name.
Some reducing reactions include:. Hydrolysis is a chemical reaction in which the addition of water splits the toxicant into two fragments or smaller molecules.
The hydroxyl group OH- is incorporated into one fragment and the hydrogen atom is incorporated into the other. Larger chemicals such as esters, amines, hydrazines, and carbamates are generally biotransformed by hydrolysis. The newest trend in the online eBook world is actually are referred to as eBook libraries, or eBook packages. An eBook package deal is something remarkable. This consists of a big number of ebooks bundled together that are not really readily available at one single place. So instead regarding hunting down and buying, say literary classics, you can buy an eBook package deal which contains all these kinds of ebooks bundled together.
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