Phospholipids of rabbit and bull sperm membranes: Structural order parameter and steady‐state fluorescence anistropy of membranes and membrane leaflets

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Abstract
The plasma (PM), outer acrosomal (OAM), and inner acrosomal membranes (IAM) were isolated from rabbit and bull spermatozoa and the major phospholipids characterized. Choline‐containing phospholipids, phosphatidylcholine (PC) and sphingomyelin (SM), constituted more than 60% of the total phospholipids (TPL) in all membranes of both species. Approximately more than 50% of PC in membrane preparations contained some form of ether linkage. Compared to OAM and IAM, cholesterol to phospholipid molar ratio was highest in PM of both species. Contrarily, protein to phospholipid ratio for PM was lowest compared to other membranes. The sphingomyelin to phosphatidylcholine ratio increased in the direction from PM to OAM to IAM. The hydrophobic fluorescent probe 1,6‐diphenyl‐1,3,5‐hexatriene (DPH) was used to examine both the steady‐state fluorescence anisotropy parameters and structural order parameter SDPH. The data showed higher rigidity in rabbit spermatozoa compared to bull spermatozoa (SDPH = 0.7582 and SDPH = 0.7326). In both species OAM had higher rigidity compared to the other two membranes (SDPH(OAM) = 0.7809, SDPH(PM) = 0.7308, and SDPH(IAM) = 0.7481 for bull; SDPH(OAM) = 0.8091, SDPH(PM)= 0.7857, and SDPH(IAM) = 0.7663 for rabbit). The inner leaflets of bull and rabbit spermatozoal membranes had significantly higher rigidity than the outer leaflets (for inner leaflet: rabbit‐SDPH(PM) = 0.8391, SDPH(OAM) = 0.8149, and SDPH(IAM) = 0.7675; bull‐SDPH(PM) = 0.8000, SDPH(OAM) = 0.7990, and SDPH(IAM) = 0.7990, and for outer leaflet: rabbit‐SDPH(PM) = 0.7021, SDPH(OAM) = 0.7145, and SDPH(IAM) = 0.6867; bull‐SDPH(PM) = 0.6986, SDPH(OAM) = 0.5980, and SDPH(IAM) = 0.7388).