SYNTHESIS, SORTING, AND PROCESSING INTO DISTINCT ISOFORMS OF HUMAN MACROPHAGE CHITOTRIOSIDASE

Chitotriosidase, the human analogue of chitinases from non-vertebrate species, has recently been identified. The macrophage-derived enzyme i s remarkably heterogeneous in molecular mass and isoelectric point. Th e synthesis and modification of the enzyme in cultured macrophages is reported. Chitotriosidase is synthesized as a 50-kDa protein with a pi of about 6.5 and 7.2. It is predominantly secreted, but in part proce ssed into a 39-kDa form with a pI of 8.0 that accumulates in lysosomes . In the C-terminal extension of the 50-kDa chitotriosidase, sialic-ac id containing O-linked glycans are present, causing its heterogeneous acidic isoelectric point. Chitotriosidase lacks N-linked glycans and t he mechanism of routing to lysosomes proves to be distinct from that o f soluble, N-glycosylated, lysosomal enzymes. It was observed that, in macrophages, alternative splicing generates a distinct chitotriosidas e mRNA species, encoding a 40-kDa chitotriosidase that is C-terminally truncated. This enzyme is almost identical to the 39-kDa chitotriosid ase formed from the 50-kDa precursor by proteolytic processing. It is concluded that the C-terminus present in the 50-kDa chitotriosidase, b ut absent in the 39-kDa isoform, was found to mediate tight binding to chitin. In the blood stream the secretory 50-kDa chitotriosidase occu rs predominantly, whilst in tissues the 39-kDa form is also abundant. These findings are consistent with the data on the synthesis and proce ssing of chitotriosidase in the cultured macrophage model.