Cathepsin D-like enzyme from embryo of camel tick Hyalomma dromedarii (Acari: Ixodida)

It is well known that cathepsin D (CD) has a fundamental role in the degradation of yolk proteins during embryo development as it has a dual function in embryo feeding and defending against pathogens. Cathepsin D-like activity is revealed during embryogenesis of Hyalomma dromedarii (Acari: Ixodida) tick and attained the highest activity in 3-4 days-old embryos during the cleavage stage and nominated camel tick embryo cathepsin D (CTECD). After purifi-cation by cationic CM-cellulose gel filtration Sephacryl S-300 columns, CTECD turned out to be homogeneous with specific activity of 40 U mg(-1) proteins, 7.4 purification factor and 55.3% yield. CTECD molecular mass was deduced from the size exclusion column to be almost 40 kDa. CTECD hydrolyzed the specific synthetic substrate Bz-Arg-Gly-Phe-Phe-Pro-4-methoxy-beta-naphthylamide confirming that the enzyme is cathepsin D. Both Zn2+ and Pepstatin A were potent inhibitors of CTECD activity. Also, CTECD displayed its maximum activity at pH 2.5 with a pH profile typical for cathepsin D. This study suggests the involvement of CTECD in yolk proteins utilization by the developing embryos as it attained its highest activity in cell cleavage stage. The study of biochemical changes during tick embryogenesis will be helpful in understanding the tick cells adaptability for different pathogens.