Isolation and identification of fetuin-B-like protein from rainbow trout seminal plasma and its localization in the reproductive system

Seminal plasma of rainbow trout (Oncorhynchus mykiss, Salmonidae) contains an inhibitory system consisting of three fractions (I-III) characterized by different electrophoretic migration rates. Using a two-step isolation procedure we purified (20- and 43-fold to homogeneity) and characterized the two subforms of inhibitor I (Ia and Ib). On the basis of the homology alignment of the amino acid sequences, inhibitor I was classified to the family of cysteine proteinase inhibitors - fetuins. The molecular masses were determined to be 61,146.5Da and 63,096.0Da, and the isoelectric points were estimated to be 6.04 and 6.22 for inhibitor Ia and Ib. Both inhibitors were glycoproteins with a carbohydrate content about 13% for inhibitor Ia and 19% for inhibitor Ib. The equilibrium association constant of inhibitor Ib with cod trypsin was determined to be 7.1×10(8)M(-1). Except for the cod trypsin inhibition, the inhibitor Ib effectively inhibited papain belonging to the cysteine proteainases. Comparative studies of the distribution of inhibitor I and the previously described inhibitor II were performed. The presence of inhibitor I in the seminal plasma was a common feature of several Salmoniformes, which was contrary to inhibitor II detected in seminal plasma of other fish families. Inhibitors I and II showed different expression patterns in the testes and spermatic duct of the rainbow trout.