Alteration in reactivity of sphingomyelinase from Streptomyces sp. modified with a polyethylene glycol derivative

Purified sphingomyelinase of Streptomyces A9107 (NRRL 15100) was modified with ss-PEG, methoxypolyethyleneglycol succinimidyl succinate, without loss of activity toward sphingomyelin in the mixed micelles with detergents such as sodium deoxycholate and Triton X-100 or toward a water-soluble, synthetic substrate, HNP, 2-(N-hexadecanoylamino)-4-nitrophenyl phosphocholine. The hemolytic and sphingomyelin-hydrolyzing activities of the enzyme toward intact bovine erythrocyte membrane were completely abolished by modification with ss-PEG, whereas the activity toward membranous sphingomyelin in the erythrocyte ghosts was fully retained. The enzyme activity toward liposomal sphingomyelin (large and small unilamellar vesicles) was significantly decreased by PEG-modification. The thermostability of sphingomyelinase was greatly increased by modification with ss-PEG, whereas the sensitivity of the enzyme to proteolysis was not influenced by that modification.