| Abstract: | Arylsulfatase B, purified to homogeneity from human eosinophils, is a tetrameric enzyme whose activity varied in accordance with the state of association of its monomeric subunits. The rate of dissociation of oligomeric forms was slow relative to the rate of the enzymatic reaction so that the kinetic properties of the enzyme depended on the concentration of the enzyme before assay. For concentrated enzyme solutions (14 micrograms/ml), Lineweaver-Burk analysis demonstrated substrate inhibition at greater than or equal to 20 mM substrate and revealed two distinct regions of activity at low and intermediate substrate concentrations. The addition of bovine serum albumin (60 micrograms/ml) or sucrose (0.25 M), which prevent subunit dissociation, yielded a linear relationship on Lineweaver-Burk analysis at non-inhibitory substrate concentrations. For dilute enzyme concentrations (4.7 micrograms/ml), inhibition occurred at greater than or equal to 2 mM substrate. Nanomolar amounts of leukotriene C4 (LTC4), relative to millimolar concentrations of substrate, inhibited eosinophil arylsulfatase B. On Lineweaver-Burk analysis, the pattern of inhibition of LTC4 with concentrated enzyme was compatible with competitive inhibition of only one oligomeric form of the enzyme, whereas at low enzyme concentrations the pattern of inhibition was apparently competitive. These findings suggest that LTC4 is a potent competitive inhibitor of a dissociated, possibly dimeric, form of the enzyme. Nanomolar concentrations of LTC4, leukotriene D4, and leukotriene E4 were equally inhibitory, whereas leukotriene B4 and isomeric 5,12-dihydroxyeicosatetraenoic acids had no inhibitory activity, indicating a requirement for a thiopeptide at C-6. Thiopeptide leukotriene analogs without an intact triene structure also lacked inhibitory activity. Sulfoxide analogs of LTC4 and leukotriene D4 were potent inhibitors, although two sulfone analogs of leukotriene D4 were not inhibitory. Arylsulfatase B did not inactivate the spasmogenic activity of sulfidopeptide leukotrienes. These findings indicate that sulfidopeptide leukotrienes and their sulfoxide derivatives may regulate by competitive inhibition the activity of oligomeric forms of the eosinophil lysosomal hydrolase, arylsulfatase B. |
