Digestive enzymes play key roles in liberating nutrients for absorption. Most animal species modulate activities of digestive enzyme activities according to the nutrient content of their diets. For example, individuals feeding on starch-rich diets tend to express higher activities of starch-degrading enzymes, and low activities of protein-degrading enzymes. This idea is known as the 'adaptive modulation hypothesis', and allows animals to digest the nutrients in their diets while still conserving biosynthetic energy and cell-membrane space.
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However, not all species modulate these activities in response to diet. For example, adult passerine birds are unable to modulate enzyme activities, and instead exhibit constitutive levels of enzyme activities that correlate with the concentration of substrates in their diets. Natural selection may favor a constitutive level of enzymes that reflects average intake of dietary substrates. We are now conducting research on rodents with different feeding strategies to investigate the flexibility of enzyme activities in response to diet composition.
In addition to nutrients, digestive enzymes must respond to a number of other components of the diet, including dietary toxins. While adaptations of digestive enzymes to toxins have been well studied in insects, little work has been done in vertebrates. My work on three disparate systems has advanced our knowledge in this area. The desert woodrat (Neotoma lepida) consumes a plant with a phenolic resin that is inhibitory to most enzymes. Woodrats increase activities of digestive enzymes in response to phenolics, presumably to overcome inhibition. The enzymes of sage grouse (specialists on sagebrush) are more tolerant than those of chickens to the toxins found in sagebrush. Finally, when spiny mice (Acomys spp.) consume fruit containing toxic glucosinolates that require activation by b-glucosidase enzymes, animals reduce activities of these enzymes to limit toxin exposure. These studies represent some of the first work investigating interactions between dietary toxins and vertebrate digestive enzymes.
In addition to nutrients, digestive enzymes must respond to a number of other components of the diet, including dietary toxins. While adaptations of digestive enzymes to toxins have been well studied in insects, little work has been done in vertebrates. My work on three disparate systems has advanced our knowledge in this area. The desert woodrat (Neotoma lepida) consumes a plant with a phenolic resin that is inhibitory to most enzymes. Woodrats increase activities of digestive enzymes in response to phenolics, presumably to overcome inhibition. The enzymes of sage grouse (specialists on sagebrush) are more tolerant than those of chickens to the toxins found in sagebrush. Finally, when spiny mice (Acomys spp.) consume fruit containing toxic glucosinolates that require activation by b-glucosidase enzymes, animals reduce activities of these enzymes to limit toxin exposure. These studies represent some of the first work investigating interactions between dietary toxins and vertebrate digestive enzymes.
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