Recombinant SMD preparations: purity and activity. The sphingomyelinase activity of pure His6 tagged recombinant toxins SMD-Ll1 and SMD-Ll2 purified from E. coli lysates was measured with a fluorescent method employing the Amplex Red reagent in a microplate reader instrument. A. SDS-polyacrylamide (10%) gel stained with Coomassie blue showing bacterial lysates and purified fractions of both SMD-Ll1 and SMD-Ll2. B. Activity curves: the fluorescence of resorufin is shown in arbitrary units (A.U.) through time. Data shown are for 100 ng SMD-Ll1; 20 ng SMD-Ll2, and 4 mU of control sphingomyelinase (Bacillus cereus).