An enzyme presenting kallikrein-like activity (designated sK1)
was
purified from the supernatant of Schistosoma mansoni
adult worm homogenate. The enzyme cleaves bradykinin from purified rat
plasma kininogen. Activity was optimal at
pH 9·0 and the enzyme showed amidolytic activity, since it
hydrolysed the kallikrein synthetic substrate d-Pro-Phe-Arg-p-nitroanilide.
The activity of sK1 upon rat plasma kininogen was strongly inhibited
by the serine proteinase inhibitors
phenylmethanesulfonyl fluoride, aprotinin or soybean trypsin inhibitor,
but not by ethylenediaminetetraacetic acid or
sodium tetrathionate. The molecular mass of sK1, as determined by
polyacrylamide gel electrophoresis in the presence
of sodium dodecyl sulfate, was 66 kDa and the pI value, estimated
by analytical chromatofocusing, was 4·2. Physical and
chemical properties suggest that sK1 is a serine proteinase of the
kallikrein family. Evidence is presented which suggests
that sK1 is a component of the tegumental surface of the parasite
and the levels of its activity in the male adult worm are
approximately 21 times higher than those in the female adult worm.
The intravenous injection of 3 μg of sK1 into an
anaesthetized rat induced a drastic reduction in the arterial blood
pressure of the animal. This effect lasted for about 1 min,
and was followed by a progressive recovery of the arterial pressure.
Neither bradycardia nor cardiac arrhythmias were
noticed, suggesting a peripheral vasodilation effect. The presence
of sK1 on the surface of adult male worms could play
an important role in the wandering capacity of coupled worms into the
visceral vasculature of the host.