The Amino‐Acid Sequence of Isoinhibitor K from Snails (Helix pomatia)

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Abstract
Performic-acid-oxidized isoinhibitor K of snails (Helix pomatia) was subjected to argininedirected tryptic proteolysis. Six peptide fragments including one overlap peptide from limited cleavage of the Arg-3-Pro-4 bond were purified to homogeneity. Four arginine peptides and the C-terminal peptide were sequenced by automatic Edman degradation using a special peptide program. The phenylthiohydantoins were all identified by chemical ionization mass spectrometry, except four cysteic acid residues that were identified on an amino acid analyzer after acid hydrolysis. Quantitative evaluation of the phenylthiohydantoins by chemical ionization mass spectrometry using total molecular-ion beam integration greatly facilitated sequencing. The mass spectrum of the dipeptide <Glu-Gly revealed that the N-terminus was blocked by pyroglutamic acid. The complete amino acid sequence of isoinhibitor K was determined. An almost 50% homology between the sequences of the snail inhibitor and the bovine trypsin-kallikrein® inhibitor (Kunitz) became obvious. A comparison of all homologous sequences of this particular class of proteins known to date is presented.

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