Adenosine triphosphate. A constituent of cholinergic synaptic vesicles

Area of Science:

• Neuroscience
• Biochemistry
• Cell Biology

Background:

• Cholinergic nerve terminals utilize synaptic vesicles for neurotransmitter storage and release.
• The precise composition and origin of molecules within synaptic vesicles are crucial for understanding neurotransmission.

Purpose of the Study:

• To investigate the presence and stability of adenosine triphosphate (ATP) within synaptic vesicles.
• To determine the relationship between vesicular ATP and acetylcholine content.

Main Methods:

• Density-gradient centrifugation was used to isolate synaptic vesicles from Torpedo electric organ extracts.
• Enzymatic assays with apyrase and myokinase assessed ATP stability.
• Radioactive labeling ([U-(14)C]ATP) traced ATP localization.
• Size-exclusion chromatography (Bio-Gel A-5m) separated vesicles from soluble components.
• Regression analysis correlated acetylcholine and ATP content.

Main Results:

• Synaptic vesicles contained significant amounts of both ATP and acetylcholine.
• Vesicular ATP demonstrated stability against enzymatic degradation, unlike free ATP.
• Radioactive ATP did not associate with vesicles during extraction, ruling out sequestration.
• Vesicular ATP and acetylcholine eluted together in size-exclusion chromatography.
• A strong positive correlation was observed between vesicular ATP and acetylcholine content (molar ratio ~5.3:1).

Conclusions:

• Synaptic vesicles actively transport and store ATP alongside acetylcholine.
• Vesicular ATP is protected from cytoplasmic enzymes, indicating a distinct intra-vesicular pool.
• The co-localization and correlated levels suggest a functional relationship between ATP and acetylcholine in these vesicles.