How Azide Ion/Hydrazoic Acid Passes Through Biological Membranes: An Experimental and Computational Study

Area of Science:

• Biochemistry
• Toxicology
• Membrane Biophysics

Background:

• Hydrazoic acid (HN3) and azide ion (N3-) (AHA) are toxic, inhibiting cytochrome c oxidase complex IV (CoX IV) in cellular respiration.
• AHA's toxicity is particularly critical in the central nervous and cardiovascular systems.
• Membrane permeability of AHA is pH-dependent due to its ionizable nature.

Purpose of the Study:

• To investigate the permeability of azide species (AHA) across biological membranes.
• To determine the rate-limiting step in AHA's toxic mechanism, specifically CoX IV inhibition.
• To correlate membrane transport rates with chemical inhibition rates.

Main Methods:

• Measured octanol/water partition coefficients of AHA at pH 2.0 and 8.0.
• Utilized Parallel Artificial Membrane Permeability Assay (PAMPA) to determine effective permeability at physiological pH.
• Validated experimental permeability data with theoretical estimations using Smoluchowski equation.

Main Results:

• Octanol/water partition coefficients were 2.01 (pH 2.0) and 0.00034 (pH 8.0).
• Effective permeability (logPe) values were -4.97 (pH 7.4) and -5.26 (pH 8.0).
• Permeation rate across cell membranes (8.46·10^4 s^-1) significantly exceeds CoX IV inhibition rate (200 s^-1).

Conclusions:

• Membrane transport of AHA is rapid and not the rate-limiting step for CoX IV inhibition.
• The rate of CoX IV inhibition is not controlled by AHA's passage through biological membranes.
• Azide poisoning dynamics are primarily governed by circulatory transport, occurring over minutes.