Transporting testosterone and its dimers by serum proteins

• The conjugation of testosterone and its dimers with serum proteins is studied here.
• Testosterone forms more stable protein adduct than its aliphatic and aromatic dimers.
• HSA forms stronger steroid conjugate than BSA.
• Hydrophobicity plays a major role in steroid–protein interaction.
• Testosterone and its aromatic dimer induced more perturbations of protein morphology than aliphatic dimer.

A substantial part of steroids is bound to serum proteins in vivo. We report the association of testosterone and it aliphatic dimer (alip) and aromatic dimer (arom) with human serum albumin (HSA) and bovine serum albumin (BSA) in aqueous solution at physiological pH. Multiple spectroscopic methods, transmission electron microscopy (TEM) and molecular modeling were used to characterize steroid–protein binding and protein aggregation process. Spectroscopic analysis showed that steroids bind protein via hydrophobic, hydrophilic and H-bonding interactions. HSA forms more stable complexes than BSA. The binding affinity of steroid–protein adducts is testosterone > dimer-aromatic > dimer-aliphatic. Transmission electron microscopy showed major changes in protein morphology as steroid–protein complexation occurred with increase in the diameter of the protein aggregate indicating encapsulation of steroids by serum proteins. Modeling showed the presence of H-bonding stabilized testosterone–protein complexes with the free binding energy of − 12.95 for HSA and − 11.55 kcal/mol for BSA, indicating that the interaction process is spontaneous at room temperature. Steroid complexation induced more perturbations of BSA conformation than HSA.

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