Lowther, Jonathan and Robinson, Mark W. and Donnelly, Sheila M. and Xu, Weibo and Stack, Colin M. and Matthews, Jacqueline M. and Dalton, John P.
The Importance of pH in Regulating the Function of the
Fasciola hepatica Cathepsin L1 Cysteine Protease.
PLoS Neglected Tropical Diseases, 3 (1).
The helminth parasite Fasciola hepatica secretes cathepsin L cysteine proteases to invade its host, migrate through tissues
and digest haemoglobin, its main source of amino acids. Here we investigated the importance of pH in regulating the
activity and functions of the major cathepsin L protease FheCL1. The slightly acidic pH of the parasite gut facilitates the
auto-catalytic activation of FheCL1 from its inactive proFheCL1 zymogen; this process was ,40-fold faster at pH 4.5 than at
pH 7.0. Active mature FheCL1 is very stable at acidic and neutral conditions (the enzyme retained ,45% activity when
incubated at 37uC and pH 4.5 for 10 days) and displayed a broad pH range for activity peptide substrates and the protein
ovalbumin, peaking between pH 5.5 and pH 7.0. This pH profile likely reflects the need for FheCL1 to function both in the
parasite gut and in the host tissues. FheCL1, however, could not cleave its natural substrate Hb in the pH range pH 5.5 and
pH 7.0; digestion occurred only at pH#4.5, which coincided with pH-induced dissociation of the Hb tetramer. Our studies
indicate that the acidic pH of the parasite relaxes the Hb structure, making it susceptible to proteolysis by FheCL1. This
process is enhanced by glutathione (GSH), the main reducing agent contained in red blood cells. Using mass spectrometry,
we show that FheCL1 can degrade Hb to small peptides, predominantly of 4–14 residues, but cannot release free amino
acids. Therefore, we suggest that Hb degradation is not completed in the gut lumen but that the resulting peptides are
absorbed by the gut epithelial cells for further processing by intracellular di- and amino-peptidases to free amino acids that
are distributed through the parasite tissue for protein anabolism.
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