Activation of human acid sphingomyelinase through modification or deletion of C-terminal cysteine.

One form of Niemann-Pick disease is caused by a deficiency in the enzymatic activity of acid sphingomyelinase. During efforts to develop an enzyme replacement therapy based on a recombinant form of human acid sphingomyelinase (rhASM), purified preparations of the recombinant enzyme were found to have substantially increased specific activity if cell harvest media were stored for several weeks at -20 degrees C prior to purification. This increase in activity was found to correlate with the loss of the single free thiol on rhASM, suggesting the involvement of a cysteine residue. It was demonstrated that a variety of chemical modifications of the free cysteine on rhASM all result in substantial activation of the enzyme, and the modified cysteine responsible for this activation was shown to be the C-terminal residue (Cys629). Activation was also achieved by copper-promoted dimerization of rhASM (via cysteine) and by C-terminal truncation using carboxypeptidase Y. The role of the C-terminal cysteine in activation was confirmed by creating mutant forms of rhASM in which this residue was either deleted or replaced by a serine, with both forms having substantially higher specific activity than wild-type rhASM. These results indicate that purified rhASM can be activated in vitro by loss of the free thiol on the C-terminal cysteine via chemical modification, dimerization, or deletion of this amino acid residue. This method of activation is similar to the cysteine switch mechanism described previously for matrix metalloproteinases and could represent a means of posttranslational regulation of ASM activity in vivo.

A biochemical and pharmacological comparison of enzyme replacement therapies for the glycolipid storage disorder Fabry disease.

Fabry disease is a lysosomal storage disease arising from deficiency of the enzyme alpha-galactosidase A. Two recombinant protein therapeutics, Fabrazyme (agalsidase beta) and Replagal (agalsidase alfa), have been approved in Europe as enzyme replacement therapies for Fabry disease. Both contain the same human enzyme, alpha-galactosidase A, but they are produced using different protein expression systems and have been approved for administration at different doses. To determine if there is recognizable biochemical basis for the different doses, we performed a comparison of the two drugs, focusing on factors that are likely to influence biological activity and availability. The two drugs have similar glycosylation, both in the type and location of the oligosaccharide structures present. Differences in glycosylation were mainly limited to the levels of sialic acid and mannose-6-phosphate present, with Fabrazyme having a higher percentage of fully sialylated oligosaccharides and a higher level of phosphorylation. The higher levels of phosphorylated oligomannose residues correlated with increased binding to mannose-6-phosphate receptors and uptake into Fabry fibroblasts in vitro. Biodistribution studies in a mouse model of Fabry disease showed similar organ uptake. Likewise, antigenicity studies using antisera from Fabry patients demonstrated that both drugs were indistinguishable in terms of antibody cross-reactivity. Based on these studies and present knowledge regarding the influence of glycosylation on protein biodistribution and cellular uptake, the two protein preparations appear to be functionally indistinguishable. Therefore, the data from these studies provide no rationale for the use of these proteins at different therapeutic doses.