BOVINE SEMINAL PLASMA ASFP - LOCALIZATION OF DISULFIDE BRIDGES AND DETECTION OF 3 DIFFERENT ISOELECTRIC FORMS

Acidic seminal fluid protein (aSFP) is a major 13 kDa protein isolated from bull seminal plasma and characterized as a new growth factor whi ch stimulates in vitro cell division and progesterone secretion by ova rian cells. Here, we establish that the four cysteines of oxidized aSF P form two disulfide bridges between nearest-neighbour residues. This pattern is conserved in boar spermadhesins, with which aSFP shares up to 50% amino acid sequence identity, and other proteins of the recentl y identified CUB domain family. Using isoelectric focusing in combinat ion with sulfhydryl group-specific blotting, the three forms of aSFP w ere identified as completely oxidized (pi 4.7), partly reduced (pi 4.8 ) and fully reduced at pi 5.1. These results indicate that native aSFP possesses two pairs of cysteine residues of different reactivity. The observation that aSFP can protect sperm from oxidative damage might b e explained by its reduction/oxidation behaviour.