Human acidic mammalian
chitinase (AMCase), a member of the family 18 glycosyl
hydrolases, is one of the important
proteins involved in Th2-mediated
inflammation and has been implicated in
asthma and allergic diseases. Inhibition of AMCase results in decreased airway
inflammation and
airway hyper-responsiveness in a mouse
asthma model, suggesting that the AMCase activity is a part of the mechanism of Th2
cytokine-driven inflammatory response in
asthma. In this paper, we report the first detailed kinetic characterization of recombinant human AMCase. In contrast with
mouse AMCase that has been reported to have a major pH optimum at 2 and a secondary pH optimum around 3-6, human AMCase has only one pH optimum for k(cat)/K(m) between pH 4 and 5. Steady state kinetics shows that human AMCase has "low" intrinsic transglycosidase activity, which leads to the observation of apparent substrate inhibition. This slow transglycosylation may provide a mechanism in vivo for feedback regulation of the
chitinase activity of human AMCase. HPLC characterization of cleavage of chitooligosaccharides (4-6-mers) suggests that human AMCase prefers the beta anomer of chitooligosaccharides as substrate. Human AMCase also appears to cleave chitooligosaccharides from the nonreducing end primarily by
disaccharide units. Ionic strength modulates the enzymatic activity and substrate cleavage pattern of human AMCase against
fluorogenic substrates, chitobiose-4-methylumbelliferyl and chitotriose-4-methylumbelliferyl, and enhances activity against chitooligosaccharides. The physiological implications of these results are discussed.