Amino Acid Composition and NH₂-Terminal Amino Acid Sequence of Rat Platelet Secretory Phospholipase A₂

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The amino acid composition and partial NH₂-terminal amino acid sequence of the phospholipase A₂ secreted by stimulated rat platelets were determined. The most predominant amino acid in the phospholipase A₂ was cysteine followed by lysine, suggesting that it is a basic one. This finding is consistent with its high affinity to a cation exchange column. The NH₂-terminal 24 amino acids were found to be as follows:
X-Leu-Leu-Glu-Phe-Gly-Gln-Met-Ile-Leu-Phe-Lys-Thr-Gly-Lys-Arg-Ala-Asp-Val-Ser-Tyr-Gly-Phe-Tyr-Gly-The enzyme contains ⁵Phe, ⁸Met, ⁹Ile, ²⁴Tyr, and ²⁵Gly residues, all of which are conserved in the sequenced pancreatic phospholipase A₂. This is the first report of the tentative characterization of a eukaryotic phospholipase A₂, the cellular source of which is known, i.e., it does not originate from a venom or the pancreas.