What is GCPII in the Small Intestine?
GCPII (glutamate carboxypeptidase II) in the small intestine functions as an intestinal folate hydrolase, a brush border enzyme that cleaves dietary polyglutamyl folates into monoglutamyl forms for absorption. 1
Primary Function: Folate Metabolism
GCPII serves as the critical enzyme for folate digestion in the small intestine. Dietary folates typically exist as polyglutamates (containing multiple glutamate residues), which cannot be absorbed directly. GCPII hydrolyzes these polyglutamyl folates at the intestinal brush border, removing glutamate residues to produce monoglutamyl folate derivatives that can then be transported across the intestinal epithelium via the reduced folate carrier (RFC) 1.
Anatomical Distribution and Expression
- Location: GCPII is expressed on the brush border membrane of small intestinal enterocytes 1, 2
- Segment-specific expression: In pigs (a relevant model for human physiology), GCPII activity is highest at birth across all small intestinal segments, becomes undetectable in the ileum after 1 week, while jejunal expression and activity predominate in adults 1
- Developmental changes: The enzyme exhibits significant ontogenic changes, with expression patterns that correlate with postnatal folate status 1
Structural Characteristics
GCPII is a heavily N-glycosylated type II membrane protein 3. The glycosylation is not merely decorative—it is essential for proper protein folding, secretion, and enzymatic activity. All predicted N-glycosylation sites are occupied, and glycosylation at sites distant from the catalytic domain remains critical for the enzyme's carboxypeptidase activity 3.
Relationship to Other GCPII Functions
While GCPII in the small intestine functions as a folate hydrolase, the same protein serves different roles in other tissues:
- Brain: Acts as N-acetylated alpha-linked acidic dipeptidase (NAALADase), cleaving NAAG to regulate glutamate neurotransmission 2, 4
- Prostate: Known as prostate-specific membrane antigen (PSMA), highly upregulated in prostate cancer 2, 5
Clinical Relevance
Genetic polymorphisms in GCPII affect folate metabolism: The 1561C→T polymorphism is associated with higher plasma folate concentrations and lower homocysteine levels, demonstrating the enzyme's physiological importance in folate homeostasis 6. This has implications for conditions where folate status is critical, including neural tube defects and cardiovascular disease.
Distinction from GCPIII
GCPIII is a close homolog of GCPII but has divergent substrate specificity. GCPIII efficiently cleaves β-citrylglutamate (BCG) while GCPII does not, suggesting distinct physiological functions 7. Importantly, cross-reactivity between GCPII-targeted therapeutics and GCPIII can cause unintended effects in tissues where GCPIII is expressed 8.