Acid phosphatase of HeLa cells: properties and regulation of lysosomal activity by serum.

Although the subcellular distribution profile of acid phosphatase in HeLa cells is typical of a lysosomal enzyme, different lysosomal (70–80%) and supernatant forms (20–30%) have been demonstrated by their differences in pH activity curves, substrate specificities, thermal stability, sensitivity to inhibitors, and kinetics. Enzymes of the lysosomal fraction displayed anomalous kinetics in the hydrolysis of p-nitrophenyl phosphate. The major lysosomal acid phosphatase activity appears to be associated with the membrane.The total acid phosphatase activity in the cell is controlled by the concentration of serum in the medium. The specific activity in the homogenates of cells grown in high serum concentration (30%) is about twice that of cells grown in low serum concentration (1%). This doubling of specific activity holds for the lysosomal enzyme (or enzymes), but little change occurs in the supernatant form (or forms). Two other lysosomal enzymes, β-glucuronidase and N-acetyl-β-d-hexosaminidase, do not increase in specific activity. The serum-dependent formation of acid phosphatase is sensitive to cycloheximide, actinomycin D, and cordycepin. Cycloheximide blocks the increase in enzymatic activity immediately, whereas cordycepin and actinomycin D have no effect for at least 8 h. These findings suggest that de novo protein synthesis is involved in the induction of lysosomal acid phosphatase by serum and that the mRNA for this enzyme is relatively stable.