In mammalian spermiogenesis, sperm mature during epididymal transit to get fertility. The pig sharing many
physiological similarities with humans is considered a promising animal model in medicine. We examined
the expression profiles of proteins from boar epididymal caput, corpus, and cauda sperm by two-dimensional
gel electrophoresis and peptide mass fingerprinting. Our results indicated that protein disulfide isomerase-
P5 (PDI-P5) human homolog was down-regulated from the epididymal corpus to cauda sperm, in contrast to
the constant expression of protein disulfide isomerase A3 (PDIA3) human homolog. To examine the
functions of PDIA3 and PDI-P5, we cloned and sequenced cDNAs of pig PDIA3 and PDI-P5 protein precursors.
Each recombinant pig mature PDIA3 and PDI-P5 expressed in Escherichia coli showed thiol-dependent
disulfide reductase activities in insulin turbidity assay. Although PDIA3 showed chaperone activity to
promote oxidative refolding of reduced denatured lysozyme, PDI-P5 exhibited anti-chaperone activity to
inhibit oxidative refolding of lysozyme at an equimolar ratio. SDS-PAGE and Western blotting analysis
suggested that disulfide cross-linked and non-productively folded lysozyme was responsible for the antichaperone
activity of PDI-P5. These results provide a molecular basis and insights into the physiological roles
of PDIA3 and PDI-P5 in sperm maturation and fertilization.