The interaction of Tb3+ with the human platelet surface

Area of Science:

• Biochemistry
• Biophysics
• Hematology

Background:

• Human platelets play a crucial role in hemostasis and thrombosis.
• Calcium ions (Ca2+) are critical regulators of platelet function.
• Characterizing calcium-binding sites on platelets is essential for understanding platelet activation and signaling.

Purpose of the Study:

• To investigate the interaction of the lanthanide terbium (Tb3+) with human platelets.
• To evaluate Tb3+ as a fluorescent probe for calcium-binding sites on the platelet surface.
• To explore the effect of platelet activation and surface modification on Tb3+ binding.

Main Methods:

• Utilized terbium (Tb3+) as a fluorescent analog for calcium (Ca2+).
• Examined Tb3+ binding kinetics and fluorescence changes on washed human platelets.
• Investigated the effects of Ca2+, La3+, plasmin treatment, and ADP activation on Tb3+ binding.
• Assessed Tb3+ competition with 45Ca2+ for platelet surface-binding sites.

Main Results:

• Tb3+ fluorescence increased ~200-fold upon binding to platelet surface proteins, likely via a Förster mechanism.
• Tb3+ binding was specific, saturable, and exhibited an apparent dissociation constant (Kd) of 195 microM.
• Plasmin treatment reduced Tb3+ fluorescence, while ADP activation increased it by 78%.
• Tb3+ effectively competed with Ca2+ for platelet surface-binding sites (IC50 ≈ 10 microM).

Conclusions:

• Tb3+ serves as a valuable fluorescent probe for characterizing Ca2+-binding sites on human platelets.
• The binding characteristics of Tb3+ are sensitive to platelet surface modifications and activation states.
• This approach offers potential for studying calcium dynamics and interactions on the platelet surface.