Immunologic Release of β-Hexosaminidase and β-Glucuronidase from Purified Rat Serosal Mast Cells

Abstract

The acid exoglycosidases β-hexosaminidase and β-glucuronidase are present in rat serosal mast cells at concentrations of 1.2 ± 0.5 units and 0.24 ± 0.11 units/106 cells, respectively, and are released in a dose- and time-dependent fashion in response to immunologic challenge with rabbit anti-rat F(ab′)2. The sum of the released and residual β-hexosaminidase and β-glucuronidase was no different from that of resting unchallenged cells, thereby indicating that release in physiologic buffer was sufficient for complete bioavailability of the secreted enzymes when assessed with small synthetic substrates. A comparison of the release of the acid exoglycosidases with that of histamine yields regression lines that have correlation coefficients of ≥ 0.97 for each enzyme and that intersect at the point of origin, suggesting absence of a threshold effect on the release of these mediators. The slopes of these regression lines indicate that at least 86 and 65% of the cellular content of β-hexosaminidase and of β-glucuronidase, respectively, are available for immunologic release and are therefore localized in mast cell secretory granules. The predominant isomeric form of β-hexosaminidase in the rat mast cell, as indicated by its chromatographic behavior, is type A and the A isomer is the predominant form released on immunologic activation of the cells. The only isomeric form of β-glucuronidase identified in rat serosal mast cells based upon its electrophoretic mobility is the lysosomal form, and this form was the only one released after immunologic activation of these cells. The mast cell secretory granule, based upon its content of acid hydrolases and its capacity to form phagolysosomes, is considered a modified lysosome, which also contains novel chemical mediators not generally observed in lysosomes of other cells.