Enzyme activity of PPT1 with disease mutations in transfected COS-1 cells. COS-1 cells were transfected with plasmids producing mutated PPT1 enzymes. The cells were lysed 48 h post-transfection and the enzyme activity of each lysate (5 μg of total protein) was analyzed by a one-hour reaction in + 37°C. The data was normalized for PPT1 expression by immunoblotting and densitometric scanning. Here the background activity is assumed to be zero. The data shown represents an average of three independent experiments and the standard deviations between these experiments are marked as error bars.