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Serum complement in the serum of two divergent Central African crocodiles, the African dwarf crocodile (Osteolaemus tetraspis) and the slender-snouted crocodile (Mecistops cataphractus) was assessed using a sheep red blood cell (SRBC) hemolytic assay. The hemolysis for both crocodilian species was serum volume-, time-, and temperature-dependent. Although the serum volume-dependent activities were similar for both species (CH50 = 81 μL for Osteolaemus tetraspis and 96 μL for Mecistops cataphractus), the kinetic curves show a greater amount cooperativity, and thus more rapid SRBC lysis, for Osteolaemus tetraspis. In addition, the hemolytic activities were very similar at 10 – 35°C, but the serum from Osteolaemus tetraspis was more active than that of Mecistops cataphractus at the temperature extremes tested (5°C and 40°C). The activities for both species were almost completely inhibited by 1 mM EDTA, indicating the dependency on divalent metal ions. However, the EDTA-inhibited hemolysis could be restored by the addition of excess Mg2+ and Ca2+, but not Ba2+, Cu2+, or Fe2+, which exhibited the specificity for Mg2+or Ca2+. These data indicate that these sympatric, but evolutionarily and ecologically divergent crocodile species have similar SRBC hemolysis activities with similar mechanisms, thus reinforcing the idea that serum complement is an ancient, innate immunity host defense system.