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 hydrola se, has been known that hydrolyses the artificial substrate 4-methylum belliferyl-tetra-N-acetylchitotetraoside (MU-[GlcNAc](4), where GlcNAc is N-acetylglucosamine). The biological function of the enzyme was un known. In this paper evidence is presented showing that this endogluco saminidase from human serum is in fact a chitinase that is different f rom lysozyme. The facts sustaining this finding are: (i) the identific ation of the products formed from MU-[GlcNAc](3) as [GlcNAc](2) and [G lcNAc](3); (ii) chitin and ethylene glycolchitin can be degraded by th e enzyme; (iii) the chitinase inhibitor allosamidin also inhibits the action of MU-TACT hydrolase from human serum; (iv) no hydrolysis of th e 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-galacto sidase, indicating a lysosomal localization for this enzyme. It is pro posed that the enzyme functions in the hydrolysis of chitin, to which mammals are frequently exposed during infection by pathogens.