CLONING AND CHARACTERIZATION OF THE SCHISTOSOMA-JAPONICUM ASPARTIC PROTEINASE INVOLVED IN HEMOGLOBIN DEGRADATION

A cDNA encoding a Schistosoma japonicum aspartic proteinase was cloned , sequenced, and found to encode a zymogen of 380 amino acid residues, and its gene was shown to be present as a single copy in the S. japon icum genome. Identity comparisons showed that the enzyme (Sjpasp) was most closely related to the cathepsin Ds. The deduced amino acid seque nce has four potential glycosylation sites, two of which are in identi cal positions to the two glycosylation sites of human kidney lysosomal cathepsin D. Furthermore, all four disulfide bonds found in mammalian cathepsin D sequences are present in Sjpasp, although the beta-hairpi n (loop 3), which is cleaved during maturation of vertebrate cathepsin Ds to yield light and heavy chain subunits, is absent from Sjpasp. Wh ile most residues involved in substrate specificity and catalysis of a spartic proteinases are preserved in Sjpasp, several residues in these regions exhibit changes that may result in a novel substrate specific ity. Aspartic proteinase activity is present in extracts of adult S. j aponicum and Schistosoma mansoni and in culture media in which schisto somes were maintained and was capable of digesting hemoglobin. The sch istosome aspartic proteinase may play a pivotal role in the catabolism of hemoglobin obtained from host erythrocytes.