Chitinases are digestive enzymes that break down glycosidic bonds in chitin. Because chitin composes the cell walls of fungi and exoskeletal elements of some animals (including worms and arthropods), chitinases are generally found in organisms that either need to reshape their own chitin or to dissolve and digest the chitin of fungi or animals.
Chitinivorous organisms include many bacteria (Aeromonads, Bacillus, Vibrio, among others), which may be pathogenic or detritivorous. They may attack arthropods, zooplankton or fungi; or they may degrade the remains of these organisms.
Fungi, such as Coccidioides immitis, are known to possess chitinases. This may be related to their typical role as detritovores and also to their potential as pathogens, infiltrating arthropods.
Plants may seem an unusual source for chitinase, but some of the archetypical chitinases have been characterized from plants (barley seed chitinase: PDB 1CNS, EC 18.104.22.168). Some plant chitinases are members of the pathogenesis related (PR) proteins which are induced after systemic acquired resistance induction (biotic and abiotic). Expression is mediated by the NPR1 gene and the salicylic acid pathway, both involved in resisting fungal and insect attack. Some may be required for creating fungal symbioses.
Chitin, like cellulose, has been thought of as abundant but difficult to digest. It is typically considered unavailable carbohydrate in animal diets, though certain fish can digest chitin to sugar; and dogma suggests that just as ruminants need bacteria to digest cellulose, chitin digestion would also require symbiosis and lengthy fermentations. As such, the discovery of animal, and particular mammalian and human chitinases is somewhat surprising. Actually, human chitinases appear in gastric juices . This is likely to be digestive chitinase, for catabolic activity.
Further, chitinase activity can be detected systemically in humans, in the blood and possibly cartilage . This is related, as is plant chitinase activity, to inflammation/pathogen resistance. As such, it is unsurprisingly related to allergies. What is surprising, perhaps, is that asthma in particular has been linked to enhanced chitinase expression levels. This may begin to explain some of the most common allergies (dust mites, mold spores - both chitin covered) and speak to the relationship between allergies and worm (helminth) infections, as part of one version of the hygiene hypothesis  (worms have chitinous mouthparts to hold the intestinal wall). Finally, the link between chitinases and salicytic acid in plants is well established - but there is a hypothetical link between salicytic acid and allergies in humans (Fiengold). The link between chitinases and allergies being now established provides a tantalizing thread that can also connect these.
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