title: High- and low-uptake forms of beta-hexosaminidase in human platelets. Selective retention of the high-uptake form during stimulation with thrombin.
abstract: Human platelets are rich in beta-hexosaminidase and other acid hydrolases contained in organelles (lysosomes) distinct from alpha-granules and dense granules. Incubation of platelets with bovine or human thrombin (100 U/ml for 5 min at 37 degrees C) induces the secretion of 100% of the contents of alpha- and dense granules, but only 40-60% of total beta-hexosaminidase from lysosomes. Both isozymes Hex A and Hex B are secreted in the same proportion as found intracellularly. There is no selective recapture or plasma membrane binding by platelets of secreted beta-hexosaminidase. The secreted enzyme is of the low-uptake type, i.e., it is poorly recognized by the phosphomannosyl receptor-mediated uptake mechanism of fibroblasts, while the retained enzyme is a 3-fold higher uptake form. Preincubation of platelets with NH4Cl (10 mM, 2 h), followed by thrombin stimulation, results in secretion of all beta-hexosaminidase as a low-uptake form. The data support the hypothesis that there are secretory and nonsecretory forms of lysosomes. The secretory lysosomes would contain low-uptake forms of hydrolases in addition to acid phosphatase, while the nonsecretory lysosomes would contain high-uptake hydrolases and be acid phosphatase-deficient. Conditions where the contents of both lysosomal populations were released together, i.e., amine treatment followed by thrombin induction, or extraction of unstimulated cells, would result in the exposure of high-uptake phosphomannosylated hydrolases released from one population of lysosomes to acid phosphatase released from the second population of lysosomes with their subsequent conversion to low-uptake forms.
mesh_terms: D001792:Blood Platelets / Q000187:drug effects / Q000201:enzymology*; D002848:Chromatography, DEAE-Cellulose; D005347:Fibroblasts / Q000378:metabolism; D054818:Hexosaminidase A; D054819:Hexosaminidase B; D006596:Hexosaminidases / Q000097:blood*; D006801:Humans; D007527:Isoenzymes / Q000097:blood*; D012497:Sandhoff Disease / Q000378:metabolism; D013917:Thrombin / Q000494:pharmacology*; D001619:beta-N-Acetylhexosaminidases
chemical_list: D007527:Isoenzymes; D006596:Hexosaminidases; D054818:Hexosaminidase A; D054819:Hexosaminidase B; D001619:beta-N-Acetylhexosaminidases; D013917:Thrombin