Identification of folate binding macromolecule in rabbit choroid plexus

Area of Science:

• Neuroscience
• Biochemistry
• Molecular Biology

Background:

• The choroid plexus plays a crucial role in regulating the cerebrospinal fluid composition.
• Folate transport into the brain is essential for neuronal function.
• Previous studies suggested specific mechanisms for folate uptake in the brain.

Purpose of the Study:

• To identify and characterize a macromolecular binder of folic acid and its derivatives in the rabbit choroid plexus.
• To investigate the properties and potential function of this folate-binding activity.

Main Methods:

• Isolation and partial purification of the binder from rabbit choroid plexus particulate fraction.
• Gel filtration (Sephadex) to determine molecular weight.
• Binding assays using radiolabeled folic acid and methyltetrahydrofolic acid.
• Inhibition studies with various folate analogs and methotrexate.
• Enzymatic treatments (ribonuclease, papain, protease) to assess binder composition.
• Comparison with dihydrofolate reductase.

Main Results:

• A macromolecular folate-binding activity was identified in the rabbit choroid plexus (2.3 nmol/g tissue).
• The binder complex had a molecular weight of 340,000-400,000 Da.
• The binder demonstrated high affinity for folic acid and methyltetrahydrofolic acid.
• Binding was inhibited by folic acid, homofolic acid, 5-methyltetrahydrofolic acid, and methotrexate.
• The binder's activity was sensitive to proteases but not ribonuclease, indicating a protein nature.
• The binder was distinct from dihydrofolate reductase.

Conclusions:

• A high-affinity folate binder exists in the rabbit choroid plexus.
• This binder is likely a protein and distinct from dihydrofolate reductase.
• The identified binder may contribute to the saturable uptake of folates by the choroid plexus.