Proteinase inhibitory activity released from the horseshoe crab blood cell during exocytosis.

The blood cell of the horseshoe crab, Limulus, is packed with granules that can be stimulated to release their contents by exocytosis. We have identified a family of proteinase inhibitors in the released materials. Included is a factor similar to the alpha 2-macroglobulin homologue present in the plasma and acid-stable and acid-instable active-site inhibitors. The acid-stable factor is active against both serine (trypsin, chymotrypsin) and metal (thermolysin) proteinases. The trypsin- and chymotrypsin-inhibitory activity has a molecular weight of 6100, as determined by gel-filtration chromatography.