Glycine improves boar sperm linear motility by promoting heme biosynthesis to regulate mitochondrial energy...

Boar sperm are prone to functional deterioration during incubation, which is commonly associated with reduced motility, impaired mitochondrial function, and insufficient adenosine triphosphate (ATP) production. This study investigated whether glycine supplementation improves boar sperm progressive and linear motility by enhancing mitochondrial energy metabolism through the 5-aminolevulinic acid (5-ALA)/heme-associated mitochondrial pathway. Boar sperm samples were incubated with different concentrations of glycine (0, 0.1, 1, and 10 mM) at 37 °C for 6 h, and sperm motility was assessed at 0, 1, 3, and 6 h. Mitochondrial membrane potential (MMP), ATP levels, 5-ALA levels, heme levels, and mitochondrial complex IV activity were also evaluated. Compared with the control group, addition of 1 mM glycine significantly increased sperm total motility, progressive motility, average path velocity (VAP), curvilinear velocity (VCL), straight-line velocity (VSL), and linearity (LIN) after 3 h of incubation (P < 0.05), whereas 10 mM glycine treatment reduced sperm motility. Glycine supplementation also significantly enhanced MMP, ATP levels, 5-ALA levels, heme levels, and complex IV activity (P < 0.05). To further evaluate the involvement of the 5-ALA/heme-associated mitochondrial pathway, exogenous 5-ALA supplementation and pharmacological inhibition of 5-aminolevulinate synthase (ALAS) and 5-aminolevulinate dehydratase (ALAD) were performed. Exogenous 5-ALA reproduced several glycine-induced improvements in sperm motility and mitochondrial energy metabolism, whereas addition of ALAS or ALAD inhibitors attenuated the beneficial effects of glycine on sperm motility, ATP levels, and heme levels (P < 0.05). In addition, artificial insemination with glycine-supplemented semen significantly increased total born litter size without significantly affecting sow pregnancy rate. These findings demonstrate that glycine improves boar sperm progressive and linear motility by enhancing mitochondrial energy metabolism. Therefore, glycine may serve as a promising metabolic additive for improving boar sperm quality and reproductive performance.