Identification of a novel acidic mammalian chitinase distinct from chitotriosidase

Chitinases are ubiquitous chitin-fragmenting hydro lases. Recently we disco vered the first human chitinase, named chitotriosidase, that is specificall y expressed by phagocytes. We here report the identification, purification, and subsequent cloning of a second mammalian chitinase. This enzyme is cha racterized by an acidic isoelectric point and therefore named acidic mammal ian chitinase (AMCase). In rodents and man the enzyme is relatively abundan t in the gastrointestinal tract and is found to a lesser extent in the lung . Like chitotriosidase, AMCase is synthesized as a 50-kDa protein containin g a 39-kDa N-terminal catalytic domain, a hinge region, and a C-terminal ch itin-binding domain. In contrast to chitotriosidase, the enzyme is extremel y acid stable and shows a distinct second pH optimum around pH 2. AMCase is capable of cleaving artificial chitin-like substrates as well as crab shel l chitin and chitin as present in the fungal cell wall. Our study has revea led the existence of a chitinolytic enzyme in the gastrointestinal tract an d lung that may play a role in digestion and/or defense.