A NOVEL HEPARIN-DEPENDENT PROCESSING PATHWAY FOR HUMAN TRYPTASE - AUTOCATALYSIS FOLLOWED BY ACTIVATION WITH DIPEPTIDYL PEPTIDASE-I

Tryptase is the major protein constituent of human mast cells, where i f is stored within the secretory granules as a fully active tetramer. Two tryptase genes (alpha and beta) are expressed by human mast cells at the level of mRNA and protein, each with a 30 amino acid leader seq uence. Recombinant precursor forms of human alpha- and beta-tryptase w ere produced in a baculovirus system, purified, and used to study thei r processing. Monomeric beta-protryptase first is shown to be intermol ecularly autoprocessed to monomeric beta-pro'tryptase at acid pH in th e presence of heparin by cleavage between Arg(-3) and Val(-2) in the l eader peptide. The precursor of ol-tryptase has an Arg(-3) to Gln(-3) mutation that precludes autoprocessing. This may explain why cr-trypta se is not stored in secretory granules, but instead is constitutively secreted by mast cells and is the predominant form of tryptase found i n blood in both healthy subjects and those with systemic mastocytosis under nonacute conditions. Second, the NH2-terminal activation dipepti de on beta-pro'tryptase is removed by dipeptidyl peptidase I at acid p H in the absence of heparin to yield an inactive monomeric form of try ptase. Conversion of the catalytic portion of beta-tryptase to the act ive homotetramer at acid pH requires heparin. Thus, beta-tryptase homo tetramers probably account for active enzyme detected in vivo. Also, p rocessing of tryptase to an active form should occur optimally only in cells that coexpress heparin proteoglycan, restricting this pathway t o a mast cell lineage.