Differences in metabolism between species may be either quantitative or qualitative, but quantitative differences are more common. In general small animals such as mice metabolize foreign compounds at a faster rate than larger animals such as humans, consistent with differences in overall metabolic rate. An extreme example of a difference in rates of metabolism is afforded by the drug oxyphenbutazone. In the dog it is rapidly metabolized and has a half-life of around 30 min, in several other species such as the rat, rabbit and monkey the half-life is between 3 and 6 h, whereas in humans metabolism is very slow and therefore the drug has a half-life of about 3 days. Quantitative differences also exist although with a few exceptions, it is generally difficult to discern useful patterns. Even the simplest organisms such as bacteria seem to be able to carry out many different types of reaction. The differences which are clear and fall within taxonomic groups are mainly found with phase 2 reactions. Differences in some cases are related to diet, and so herbivores and carnivores may show differences.
Examples of toxicologically important species differences in metabolism will therefore be dealt with by considering the different types of metabolic reactions.
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