Proteolytic processing of human lysosomal arylsulfatase A.

Arylsulfatase A purified from human placenta contained an unreported component with an apparent molecular mass of 7 kDa in addition to the two known components with apparent molecular masses of 58 and 50 kDa. The detailed relationship between the 58 kDa component and the 50 kDa component is as yet unknown. The present study was undertaken to define the structure of the subunits of the sulfatase. The N-terminal sequence of the 50 kDa component was identical to that of the 58 kDa component. Furthermore, the peptide maps of the 50 kDa component, which was separately digested with trypsin and Achromobacter proteinase I, were quite similar to those of the 58 kDa one. Through sequence analysis of the incompatible peaks in the peptide maps, the 50 kDa component was found to lack a sequence from Val-445 to the C-terminus. On the other hand, the N-terminal sequence of the 7 kDa component began with Ala-448, though there was a minor sequence commencing with Thr-449. These observations suggest that the 50 and 7 kDa components were produced by limited proteolysis near the C-terminus of the 58 kDa component. Through analysis using unreducing SDS-PAGE, the 58 and the 7 kDa components were found to be linked by disulphide bonds. Arylsulfatase A purified from human liver was also composed of the same subunits as the placental one. This finding suggests that human arylsulfatase A undergoes similar proteolytic processing regardless of the tissue involved.