Characterisation of the carboxylesterase enzyme cauxin in the seminal fluid of the cat

The carboxylesterase cauxin is a major urinary protein in cats that is also found in seminal fluid (SF). This study investigated cauxin in feline SF including biochemical features, concentration, distribution and gene expression in epididymal tissue, and its reaction with acylglycerol substrates.Monomeric, dimeric, and/or multimeric forms of cauxin carrying N-glycosylations were detected on Western blots of feline SF but most were monomeric. Cauxin concentrations were markedly lower in SF (0.042 ± 0.020 mg/mL) than in urine (∼0.5 mg/mL) and cauxin gene expression was 60-fold lower in the epididymis than in the kidney. Immunohistochemical examination localised cauxin within the stereocilia and cytoplasm of epithelial cells lining the caput and corpus epididymis. Cauxin-positive spermatozoa were detected in the lumen of the cauda epididymis but not in the cytoplasm of the epithelial cell lining. Using an in vitro assay, cauxin hydrolysed saturated 1-mono- but not di- and tri-acylglycerols. The results suggest that cauxin secreted from the caput and corpus epididymis acts as an esterase on lipid within feline SF.