On the biological role of a proteolytic-enzyme inhibitor from the ectoparasitic tick Boophilus microplus.

The tick Boophilus microplus contains a protein that inhibits a range of proteolytic enzymes. Variations in the concentration of this protein throughtout the life cycle were followed by measuring simultaneously the inhibition of trypsin and chymotrypsin and reaction with an antiserum to the purified inhibitor. The protein is present in large amounts in eggs and in unfed larvae, but its concentration falls very rapidly after the start of the parasitic stage of the life cycle. This, together with previous evidence, suggests that the inhibitor is important both in eggs and in the initial establishment of the parasite on its host. The activity of the protein towards several enzymes has been measured as an indication of its possible function. Bovine trypsin, chymotrypsin and plasmin and pig pancreatic kallikrein are all inhibited. The protein also affects the blood-coagulation system at several points, since it prolongs both activated-partial-thromboplastin time and prothrombin time. It inhibits the complement-dependent lysis of erythrocytes, but is without significant effect on mitogen-induced lymphocyte stimulation. Thus the inhibitor could have several effects on the host that would be beneficial to the parasite.