Аннотация:The red king crab,P. camtschaticus, is a large commerciallyimportant crustacean in Russia, Norway, and the US, withannual landings estimated in the range of tens of millions of kgs.Previously, from theP. camtschaticuswe isolated a serpin (fur-ther serpin PC), which had an anticoagulant and an anticomple-ment effects on the human blood. Based on its action wehypothesized its role in host’s immunity since these proteins werereported to be engaged in defense systems of other crustaceans,are highly homologous, and there is still a lack of valuable bio-chemical data on their exact role, partner peptidases, and speci-ficity. Recombinant serpin PC’s reactivity was previously testedtowards various serine and cysteine proteases, and it was foundthat it inhibits a bovine cationic trypsin, the reaction proceeds hrough the characteristic serpin mechanism, with the unusualreaction site. After we proved its inhibitory function on themodel partner peptidase, we decided to find whether it performsas the inhibitor in a host organism and clarify its function. Ear-lier we found that its highest level of transcription is in the hemo-cytes, and, due to the presence of the secretion signal, it can besecreted to the hemolymph. Thus, we performed a peptidaseactivity profiling of hemocytes and plasma with the recombinantserpin PC and other peptidase inhibitors, and found that it inhi-bits at least two R/K-specific activities. In arthropods, hemocyteserpins are known to regulate the melanization cascade, thatinvolves a number of peptidases of aforementioned specificity,therefore we further assessed serpin PC’s effect on its activation.We showed that it twice inhibits the induction of phenoloxidaseactivity, through which the melanization manifests. As a result,we approached further to the identification of serpin PC’s nativetargets and function. The research was carried out within theframework of RSCF project No. 16-14-00191