Predominance of Biliverdin over Bilirubin in Human Seminal Plasma

Biliverdin (BV) and bilirubin (BR) are established endogenous antioxidants and immune modulators in other organ systems; however, their roles in the male genital tract remain undefined. The aim of this study was to quantify both bile pigments in human seminal plasma using a fluorescent protein biosensor and to examine their associations with basic semen parameters. We analyzed forty-two semen samples from men undergoing infertility evaluation. Biliverdin predominated over bilirubin in 88.1% of samples. Biliverdin concentration ranged from 51.8 to 611.2 nM, whereas bilirubin ranged from 19.7 to 240.7 nM. The mean total amounts per ejaculate were 1054 pmol for biliverdin and 280 pmol for bilirubin. The total amount of bilirubin in the ejaculate was positively correlated with total sperm count (Rs = 0.47; p = 0.028), whereas biliverdin showed no significant association (Rs = 0.21; p = 0.723). Oligozoospermic samples had significantly lower bilirubin concentrations (p < 0.001) and lower total bilirubin amounts (p < 0.005). Teratozoospermic samples exhibited significantly higher biliverdin concentrations (p < 0.05). This study provides the first simultaneous quantification of biliverdin and unconjugated bilirubin in human seminal plasma and identifies distinct associations with sperm quality. These findings suggest that bile pigments may reflect localized redox-related processes in the male genital tract and may influence male fertility potential.