Involvement of apolipoprotein A in maintaining tissue fluid balance in goldfish Carassius auratus.

Mammalian plasma proteins play a key role in maintaining tissue fluid balance because they are retained within capillaries and thus create colloid osmotic pressure. Likewise, fish plasma contain a considerable concentration oligomeric proteins which likely serve a similar role. To elucidate the functions of these oligomeric proteins, we analyzed blood serum (BS) and interstitial fluid (IF) complexes in goldfish from the wild and under experimental conditions using 2D electrophoresis and matrix-assisted laser desorption/ionization (MALDI). We detected protein compounds with MWs ranging from 50 to 155 kDa, organized as oligomeric complexes. The protein compounds consisted of apolipoproteins АроА-I and Аро-14 which are homological to mammalian АроА-I and АроА-II, respectively. The 155-kDa and 50-125-kDa oligomer complexes were located very low-density lipoproteins (LDL) and high-density lipoproteins (HDL) areas on the BS/IF proteomic maps, respectively. The latter resembled mammalian HDL plasma particles by size and contained lipids, so we considered them as HDL particle populations. Investigation of the uniform dissociation/association mechanism for HDL and LDL oligomers in goldfish, from the wild and under critical salinity conditions, showed the "125/110 → 85/60 kDa" reorganization. This was associated with overcoming physiological stress during spawning and under critical salinity conditions. Opposite reorganization "85/60 → 125/110 kDa" was associated with restoration of metabolic processes after stress. The association/dissociation reorganizations promoted equilibration of BS and IF osmolarities in all fish groups. We discuss the connection of these reorganizations with total protein distribution across the capillary wall and salinity, as well as the role of oligomeric apolipoproteins as universal metabolic regulators.