ZAG and Zn/ZAG Ratio in Seminal Plasma: Dual Roles in Male Semen Quality

Zinc-alpha2-glycoprotein (ZAG), a multifunctional glycoprotein abundant in seminal plasma, may influence sperm function through lipid metabolism regulation, cAMP signaling, and zinc-ion homeostasis. This study systematically evaluated the association between seminal plasma ZAG levels and semen parameters (sperm concentration and motility) and explored its clinical implications. A total of 337 participants (excluding zinc supplement users and individuals with reproductive disorders) were enrolled. Semen parameters were assessed via computer-assisted semen analysis (CASA), seminal plasma zinc levels were measured using chemiluminescence, and ZAG concentrations were quantified via ELISA. Stratified analyses were performed by age, BMI, semen parameters, and zinc levels, and correlations were evaluated using multivariable linear regression, restricted cubic spline (RCS) analysis, and threshold-effect models. Results showed that, in stratified analyses, ZAG concentrations were significantly higher in men aged > 35 years, those who were overweight/obese (BMI ≥ 25kg/m²), and the high-zinc subgroup (all P < 0.05). The lowest ZAG level was observed in the oligozoospermia group (377.69 µg/mL), whereas the highest was found in the oligoasthenozoospermia group (833.82 µg/mL). In correlation analyses, seminal plasma ZAG was positively associated with sperm concentration (β = 0.01, 95% CI: 0.01-0.02, P < 0.001), sperm concentration was increased in the highest ZAG quartile (Q4) compared with the lowest quartile (Q1)(P< 0.001). Conversely, ZAG was negatively associated with sperm motility (β = - 0.01, 95% CI: - 0.99 to - 0.01, P < 0.001), with Q4 showing a reduction compared with Q1(P = 0.017). Notably, the Zn/ZAG ratio displayed a threshold effect on motility (inflection point = 0.70): below this point the ratio correlated positively with motility (β = 17.55, P = 0.002), whereas above it the correlation became negative (β = - 39.52, P = 0.023). The results indicate a significant positive correlation between ZAG concentration and sperm concentration, and a negative correlation with sperm motility. Furthermore, the Zn/ZAG ratio exhibits a threshold effect on sperm motility, highlighting the interaction between zinc homeostasis and ZAG in regulating sperm function. These findings enhance our understanding of the role of ZAG in male reproductive physiology and suggest its potential as a biomarker for semen quality assessment.