Adipsin is serine protease that is secreted by adipocytes. It is deficient in several animal model of obesity. Adipsin has now been identified as the same protein as complement factor D. Adipsin, also called ADN or complement factor D or C3 convertase activator or properdin factor D (precursors: mouse 259-aa; rat 263 aa, human 253 aa, mature protein 26-253, ~22kD) cleaves factor B when the latter is complexed with factor C3B, activating the C3BB complex, which then becomes C3 convertase of the alternative pathway. Adipocyte is the major protein secreted by the adipocytes. Unlike rodents, adipsin is also expressed in monocytes/macrphages. Most adipsin is secreted in blood (50ug/ml in normal lean mice and 50-100 fold less in fat from db/db or ob/ob or MSG (monosodium glutamate-treated mice). Its expression is induced upon differentiation of preadipocytes.