Dependence of App1p PAP activity on the surface concentrations of PA, DGPP, and LPA. Activity was measured as a function of the indicated surface concentration of PA, DGPP, or LPA. The molar concentrations of the substrates were held constant at 2 mm, and the Triton X-100 concentrations were varied to obtain the indicated surface concentrations. The amount of phosphate released from the substrates in the enzyme reactions was determined by the colorimetric assay using the malachite green-molybdate reagent. The data shown are means ± S.D. (error bars) from triplicate determinations.