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 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 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 glycans and the mechanism of routing to lysosomes proves to be distinct from that of soluble, N-glycosylated, lysosomal enzymes. It was observed that, in macrophages, 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.
Renkema
G H
GH
Department of Biochemistry, University of Amsterdam, Academic Medical Center, The Netherlands.
Boot
R G
RG
Strijland
A
A
Donker-Koopman
W E
WE
van den Berg
M
M
Muijsers
A O
AO
Aerts
J M
JM
D000818:Animals; D002478:Cells, Cultured; D004792:Enzyme Precursors / Q000096:biosynthesis / Q000737:chemistry / Q000378:metabolism; D005776:Gaucher Disease / Q000201:enzymology; D006031:Glycosylation; D006596:Hexosaminidases / Q000096:biosynthesis* / Q000737:chemistry / Q000378:metabolism; D006801:Humans; D007526:Isoelectric Point; D007527:Isoenzymes / Q000096:biosynthesis* / Q000737:chemistry / Q000378:metabolism; D008247:Lysosomes / Q000201:enzymology; D008264:Macrophages / Q000201:enzymology* / Q000648:ultrastructure; D013058:Mass Spectrometry; D016253:Microscopy, Immunoelectron; D008970:Molecular Weight; D011499:Protein Processing, Post-Translational
D004792:Enzyme Precursors; D007527:Isoenzymes; D006596:Hexosaminidases; C086400:chitotriosidase