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 is remarkably heterogeneous in molecular mass and isoelectric point. The synthesis and odification 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 processed 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‐acid containing O‐linked glycans are present, causing its heterogeneous acidic isoelectric point. Chitotriosidase lacks N‐linked lycans and the mechanism of routing to lysosomes proves to be distinct from that of soluble, N‐glycosylated, lysosomal enzymes. It was observed that, in acrophages, alternative splicing generates a distinct chitotriosidase mRNA species, encoding a 40‐kDa chitotriosidase that is C‐terminally truncated. This enzyme is almost identical to the 39‐kDa chitotriosidase formed from the 50‐kDa precursor by proteolytic processing. It is concluded that the C‐terminus present in the 50‐kDa chitotriosidase, but absent in the 39‐kDa isoform, was found to mediate tight binding to chitin. In the blood stream the secretory 50‐kDa chitotriosidase occurs predominantly, whilst in tissues the 39‐kDa form is also abundant. These findings are consistent with the data on the synthesis and processing of chitotriosidase in the cultured macrophage model.