A calcium-stimulated alkaline phosphatase associated with synaptic vesicles

Area of Science:

• Biochemistry
• Neuroscience
• Enzymology

Background:

• Synaptic vesicles are crucial for neurotransmission.
• Understanding vesicle-associated enzymes aids in elucidating synaptic function.
• Alkaline phosphatases play diverse roles in cellular processes.

Purpose of the Study:

• To characterize the alkaline phosphatase activity within isolated bovine cerebral cortex synaptic vesicles.
• To differentiate this activity from other phosphatases and ATPases in the brain.
• To investigate the enzymatic properties and potential function of synaptic vesicle alkaline phosphatase.

Main Methods:

• Isolation of synaptic vesicles from bovine cerebral cortex.
• Assay of alkaline phosphatase activity using p-nitrophenylphosphate and alpha-naphthyl phosphate.
• Determination of molecular weight, optimal pH, and kinetic parameters (Km).
• Enzyme inhibition studies using various agents (EGTA, Ca2+, Mg2+, specific inhibitors).
• Comparison with ATPase activity and nucleotide hydrolysis.

Main Results:

• Synaptic vesicles exhibit alkaline phosphatase activity, distinct from a major brain homogenate phosphatase (MW 140,000 vs. 125,000).
• The enzyme's optimal pH is 9.8, with a Km of 0.31 mM for p-nitrophenylphosphate.
• Phosphatase activity is Ca2+-dependent and inhibited by EGTA, differing from Ca2+/Mg2+-dependent ATPase activity.
• Trypsin digestion reduced phosphatase activity but retained ATPase activity, indicating distinct enzymes.
• The enzyme showed no diesterase activity but was inhibited by ATP, ADP, and AMP, suggesting complex nucleotide metabolism.

Conclusions:

• Bovine synaptic vesicles contain a unique alkaline phosphatase, separable from major brain phosphatases and ATPases.
• The enzyme's Ca2+ dependency and distinct kinetic properties suggest a specific role within the synaptic vesicle.
• Further research is needed to fully elucidate the function of this synaptic vesicle-associated alkaline phosphatase.