Identification and Characterization of Blood Serum Proteins with High Osmotic Activity in the Pike Esox lucius

Abstract The search and identification of blood serum proteins with high osmotic activity (osmotically active proteins, OAP) were carried out in albumin-containing fish, northern Esox lucius L., using electrophoresis and MALDI mass spectrometry. Several proteins with high negative charge (8 extracellular and 1 intracellular) were found and identified in the pike blood serum. The total relative content (TRC) of all OAP was ~60%. The relative content (RC) of individual OAP—hemopexin, proteinase inhibitors, apopoprotein A-I (in high density lipoproteins) and albumin—were ~30, ~10, ~12, and 3.6%, respectively; intracellular protein Grb14 was represented by trace amounts. According to the annotations of Gene Ontology, the main functions of OAP are related to the protection and transport. At the same time, the high negative charge of OAP suggests their high osmotic activity. The comparison of OAP lists in albumin-containing pike and albumin-free fishes shows that they are consistent across all extracellular OAP except for albumin, which is negligible in the pike serum. In light of the albumin-free model of capillary exchange, these facts do not point to a key role of albumin in osmotic homeostasis control in fish. The multiplicity of OAP in the blood of teleost fishes distinguishes them from mammals, where serum albumin is a key factor controlling capillary fluid exchange.