Search and identification of serum proteins with high osmotic activity in pike <i>Esox lucius</i>

The search and identification of proteins with high osmotic activity (OAPs) in the blood serum of a representative of albumin-containing teleost fish — pike Esox lucius L. were carried out using 2D-electrophoresis and MALDI mass-spectrometry. Using the criterion of high negative charge of proteins in disk-electrophoresis, 8 extracellular and one intracellular OAPs were identified. Their total relative content was ~60% of the total serum protein concentration: ~30% for hemopexin, ~10 and ~12% for proteinase inhibitors and apolipoprotein A (in high-density lipoprotein composition), respectively, 3.6% for albumin and "traces" of intracellular Grb14. According to gene ontology annotations, the main functions of OAPs are associated with protection and transport, and the manifestation of high osmotic activity of OAPs is due to their high negative charge. Comparison of the list of OAPs in albumin-containing pike with the list of OAPs in albumin-free teleost fish indicates their coincidence in all extracellular proteins except albumin. In light of the albumin-free model of capillary exchange, this fact suggests an ordinary, rather than a key role of albumin in the control of osmotic homeostasis inside the body. The multiplicity of OAPs in the blood of teleost fish distinguishes them from mammals, in which serum albumin specializes in the control of capillary fluid exchange.