Isoquinoline alkaloids. Inhibitory actions on cation-dependent ATP-phosphohydrolases

Area of Science:

• Biochemistry
• Neuroscience
• Pharmacology

Background:

• Isoquinoline alkaloids are a diverse group of natural compounds.
• Na+, K+-ATPase and Mg2+-ATPase are crucial enzymes in brain function.
• Understanding enzyme inhibition by alkaloids is important for pharmacology.

Purpose of the Study:

• To investigate the inhibitory effects of various isoquinoline alkaloids on rat brain microsomal ATPases.
• To determine the relative potency of different alkaloid classes.
• To characterize the kinetic mechanisms of inhibition for specific alkaloids.

Main Methods:

• Preparation of rat brain microsomal fractions.
• Assays for Na+, K+-ATPase and Mg2+-ATPase activity.
• Kinetic analysis of enzyme inhibition using double reciprocal plots.
• Measurement of K+-activated p-nitrophenylphosphatase (pNPPase) activity.

Main Results:

• Eleven classes of isoquinoline alkaloids inhibited both Na+, K+-ATPase and Mg2+-ATPase.
• Na+, K+-ATPase was generally more sensitive to inhibition.
• Protoberberines were the most potent inhibitors, followed by benzophenanthridines, benzylisoquinolines, and aporphines.
• Berberine and 1,2,10,11-tetrahydroxyaporphine showed competitive inhibition with respect to ATP.
• Sanguinarine and papaveroline exhibited noncompetitive inhibition with respect to ATP.
• All tested alkaloids showed non-linear competitive inhibition concerning Na+-activation.
• Inhibition of K+-activated pNPPase was also observed.

Conclusions:

• Certain isoquinoline alkaloids significantly inhibit various cation-dependent phosphohydrolases in the rat brain.
• The specific class and chemical structure of the alkaloid influence its inhibitory potency and mechanism.
• These findings provide insights into the molecular targets of isoquinoline alkaloids in the central nervous system.