Low molecular weight serine protease inhibitors from insects are proteins with highly conserved sequences

A low molecular weight protease inhibitor peptide found in ovaries of the desert locust Schistocerca gregaria (SGPI-2), was purified from plasma of the same locust and sequenced. It was named SGCI. It was found active towards chymotrypsin and human leukocyte elastase. SGCI was synthesized using a solid-phase procedure and the sequence of its reactive site for chymotrypsin was determined. Compared with an inhibitor purified earlier from another locust species, the total sequence of SGCI showed 88% identity. In particular, the sequence of the reactive site of these inhibitors was identical. Our search for a closely related peptide in an insect species far removed from locusts, the lepidopteran Spodoptera littoralis, was unfruitful but a different chymotrypsin inhibitor, belonging to the Kazal family, was found whose mass is greater than that of SGCI (20 vs 3.6 kDa). Its N-terminal sequence shares 80% identity with that of a chymotrypsin inhibitor purified earlier from the haemolymph of another lepidopteran. Conservation of the amino acid sequence in the reactive site seems to be an exception among protease inhibitors.