Differential release of membrane-bound alkaline phosphatase isoenzymes from tumor cells by bromelain.

A rapid enzymatic method is presented which results in the selective release of cell-surface alkaline phosphatase isoenzymes. Treatment of suspensions of human tumor cell lines with the proteolytic enzyme bromelain released certain alkaline phosphatase isoenzymes into low-molecular-weight, catalytically active forms. Cells which expressed term placental or intestinal isoenzymes were equally susceptible to this treatment. A cell line which expressed early placental isoenzyme, however, was unaffected by bromelain as indicated by complete recovery of activity in the treated membrane fraction. Successful solubilization of immunologically reactive enzyme allows quantitation of levels of cell-surface enzyme in response to modulators of gene expression. Moreover, the observed selective solubilization of isoenzymes by bromelain may be of general use in analyses of the physical association between other biologically important surface proteins and the lipid components of the cell membrane.