Plasma protein binding and endothelial enzyme interactions in the lung

Area of Science:

• Biochemistry
• Pharmacology
• Physiology

Background:

• Plasma protein binding, particularly albumin binding, can influence the availability and metabolism of drug substrates.
• Angiotensin-converting enzyme (ACE) plays a crucial role in the renin-angiotensin system and is found in pulmonary endothelium.
• The synthetic ACE substrate [3H]benzoyl-phenylalanyl-alanyl-proline (BPAP) is used to study ACE activity.

Purpose of the Study:

• To investigate the impact of plasma albumin binding on the hydrolysis of the synthetic ACE substrate BPAP by pulmonary endothelial ACE.
• To determine the influence of binding kinetics between BPAP and bovine serum albumin (BSA) on BPAP hydrolysis in isolated rabbit lungs.

Main Methods:

• Isolated rabbit lungs were perfused with a salt solution containing either 5% bovine serum albumin (BSA) or 5% dextran.
• The single-pass indicator-dilution method was employed to measure the fraction of [3H]BPAP hydrolyzed (M).
• The effect of varying contact time (ti) between BPAP and BSA before lung perfusion was assessed.

Main Results:

• Lung hydrolysis (M) of [3H]BPAP was significantly higher in albumin-free perfusate compared to BSA-containing perfusate.
• Increasing the contact time (ti) between BPAP and BSA led to a decrease in lung hydrolysis (M).
• Kinetic modeling revealed slow binding interactions between BPAP and BSA, with a dissociation rate constant of approximately 0.015 s-1 and 22% of BPAP bound at equilibrium.

Conclusions:

• Transient plasma protein binding kinetics, specifically albumin binding, can significantly affect the hydrolysis of ACE substrates like BPAP by pulmonary endothelial ACE.
• The rate and extent of BPAP-albumin binding influence its availability for enzymatic hydrolysis.
• These findings highlight the importance of considering protein binding dynamics in the pharmacokinetic and pharmacodynamic assessment of ACE substrates.