Human serum contains a chitinase: identification of an enzyme, formerly described as 4-methylumbelliferyl-tetra-N-acetylchitotetraoside hydrolase (MU-TACT hydrolase).

Since 1988 an endoglucosaminidase, provisionally named MU-TACT hydrolase, has been known that hydrolyses the artificial substrate 4-methylumbelliferyl-tetra-N-acetyl-chitotetraoside (MU-[GlcNAc]4, where GlcNAc is N-acetylglucosamine). The biological function of the enzyme was unknown. In this paper evidence is presented showing that this endoglucosaminidase from human serum is in fact a chitinase that is different from lysozyme. The facts sustaining this finding are: (i) the identification of the products formed from MU-[GlcNAc]3 and [GlcNAc]2;and [GlcNAc]3; (ii) chitin and ethylene glycolchitin can be degraded by the enzyme; (iii) the chitinase inhibitor allosamidin also inhibits the action of MU-TACT hydrolase from human serum; (iv) no hydrolysis of the lysozyme substrate Micrococcus lysodeikticus. The enzyme also occurs in rat liver. It was demonstrated that upon Percoll density gradient centrifugation the enzyme from this tissue distributed parallel to the lysosomal marker enzymes beta-N-acetylhexosaminidase and beta-galactosidase, indicating a lysosomal localization for this enzyme. It is proposed that the enzyme functions in the hydrolysis of chitin, to which mammals are frequently exposed during infection by pathogens.