Proteins present in and secreted into the lumen of various regions of the stallion epididymis were characterized qualitatively and quantitatively by two-dimensional electrophoresis. Using this proteomic approach, 201 proteins were found in the lumen and 117 were found that were secreted by the epithelium in various parts of the organ. Eighteen proteins made up 92.6% of the total epididymal secretory activity, lactoferrin (41.2%) and clusterin (24.8%) being the most abundant. Procathepsin D, HE1/CTP (cholesterol transfer protein), GPX (glutathione peroxidase), beta-N-acetyl-hexosaminidase, and PGDS (prostaglandin D2 synthase) were the other major compounds secreted. The most abundant proteins found in the luminal fluid were albumin and the secreted proteins: lactoferrin, PGDS, GPX, HE1/CTP, and hexosaminidase.
Three main secretory epididymal regions were identified from the protein pattern, i.e., regions E0–E2, E3–E5, and E6–E9. Region E0–E2 was characterized by the secretion of clusterin (53%), PGDS (44%), and GPX (6%). Region E3–E5 had the highest number of secreted proteins, the highest protein concentrations (60–80 mg/ml), and the highest spermatocrit value (85%). Lactoferrin (60% in E4), clusterin (29% in E3), hexosaminidase (10% in E3), and procathepsin D (6.9% in E4) were the most abundant proteins in this region. Region E6–E9, in which few region-specific secreted compounds were found, was characterized by a high quantity of lactoferrin in the luminal fluid (2–14 mg/ml). Comparison between the secretion of the major proteins and their concentrations in the lumen throughout the organ showed that the behavior of each protein is specific, in particular for the three isoforms of clusterin.