Lysophospholipase from the human blood fluke, Schistosoma japonicum

Abstract

Background: Given the unusual nature of the schistosome surface (a highly unusual lipid bi-layer) and the central role of the schistosome tegument in host-parasite relations, an enhanced understanding of the lipid biochemistry of the schistosome surface can be expected to provide new insights into schistosome pathogenesis and lead to new interventions.

Methods: Bioinformatics approaches including three-dimensional homology modeling, along with recombinant expression, dimensional gel electrophoresis, immunoblotting, and Southern hybridizations were employed to characterize a novel lysophospholipase gene transcript from Schistosoma japonicum.

Results: A transcript encoding a small form lysophospholipase from the egg stage of S. japonicum was isolated as an expressed sequence tag (EST). The deduced polypeptide included 227 amino acid residues, shared identity with lysophospholipases of Schistosoma mansoni and Rattus norvegicus, and esterase A of Pseudomonas fluorescens, appeared to belong to the abhydrolase_2 family of phospholipases and carboxylesterases, and was structurally related to the alpha/beta-hydrolases (pfam00561). The S. japonicum enzyme exhibited the GXSXG consensus active site characteristic of serine proteases, esterases, and lipases, and included the catalytic triad motif of Ser-Asp-His residues characteristic of serine hydrolases. Three-dimensional structural predictions accomplished using the coordinates of human acyl protein thioesterase and P. fluorescens esterase indicated that the putative catalytic triad formed by these three residues was located at the alpha/beta-hydrolase fold characteristic of the lipases and esterases. Soluble S. japonicum lysophospholipase was expressed in Escherichia coli as a recombinant enzyme of approximately 26kDa and employed to raise a mono-specific antiserum. Immunoblot analysis revealed a single 23-kDa band in both membrane-associated and soluble tissue fractions of adult schistosomes. Southern hybridization and bioinformatics analyses indicated the likely presence of allelic-specific polymorphisms and/or two copies of the lysophospholipase gene in the S. japonicum genome.

Conclusions: A small form lysophospholipase has been characterized from the human schistosome, S. japonicum. The availability of the recombinant S. japonicum lysophospholipase should facilitate further characterization of the enzyme, including its substrate and inhibition profiles and its potential as an interventional target. Schistosome lysophospholipase may represent a new target for anti-schistosomal chemotherapy given that metrifonate, which targets the related enzyme acetylcholinesterase, is an effective and safe medicine for treatment of urinary schistosomiasis.