Abstract
β-Citrylglutamate (BCG), a compound present in adult testis and in the CNS during the pre- and perinatal periods is synthesized by an intracellular enzyme encoded by the RIMKLB gene and hydrolyzed by an as yet unidentified ectoenzyme. To identify β-citrylglutamate hydrolase, this enzyme was partially purified from mouse testis and characterized. Interestingly, in the presence of Ca(2+), the purified enzyme specifically hydrolyzed β-citrylglutamate and did not act on N-acetyl-aspartylglutamate (NAAG). However, both compounds were hydrolyzed in the presence of Mn(2+). This behavior and the fact that the enzyme was glycosylated and membrane-bound suggested that β-citrylglutamate hydrolase belonged to the same family of protein as glutamate carboxypeptidase 2 (GCP2), the enzyme that catalyzes the hydrolysis of N-acetyl-aspartylglutamate. The mouse tissue distribution of β-citrylglutamate hydrolase was strikingly similar to that of the glutamate carboxypeptidase 3 (GCP3) mRNA, but not that of the GCP2 mRNA. Furthermore, similarly to β-citrylglutamate hydrolase purified from testis, recombinant GCP3 specifically hydrolyzed β-citrylglutamate in the presence of Ca(2+), and acted on both N-acetyl-aspartylglutamate and β-citrylglutamate in the presence of Mn(2+), whereas recombinant GCP2 only hydrolyzed N-acetyl-aspartylglutamate and this, in a metal-independent manner. A comparison of the structures of the catalytic sites of GCP2 and GCP3, as well as mutagenesis experiments revealed that a single amino acid substitution (Asn-519 in GCP2, Ser-509 in GCP3) is largely responsible for GCP3 being able to hydrolyze β-citrylglutamate. Based on the crystal structure of GCP3 and kinetic analysis, we propose that GCP3 forms a labile catalytic Zn-Ca cluster that is critical for its β-citrylglutamate hydrolase activity.
Highlights
Increases during sexual maturation and remains constant during adulthood [1,2,3]
NAAG is hydrolyzed into N-acetyl-aspartate (NAA) and glutamate by glutamate carboxypeptidase 2 (GCP2), a membrane-bound, glycosylated ectoenzyme with its catalytic site oriented toward the extracellular environment, in good agreement with the observation that NAAG can be released from neurons [12]
GCP2 was designated as FOLH1 because of its hydrolase activity on folyl-polyglutamate, and as prostate specific membrane antigen (PSMA) as it is highly expressed in prostate cancer [13,14,15]
Summary
Increases during sexual maturation and remains constant during adulthood [1,2,3]. the exact physiological function of BCG is presently unknown, different observations suggest that it may play an important role during brain development and spermatogenesis [4, 5]. NAAG is secreted by neurons upon calcium-dependent depolarization and has long been thought to bind to the glutamate metabotropic receptor mGluR3, possibly attenuating the glutamate-induced excitotoxicity [8]. This neurotransmitter function of NAAG is still debated (9 –11). NAAG is hydrolyzed into N-acetyl-aspartate (NAA) and glutamate by glutamate carboxypeptidase 2 (GCP2), a membrane-bound, glycosylated ectoenzyme with its catalytic site oriented toward the extracellular environment, in good agreement with the observation that NAAG can be released from neurons [12]. RIMKLA synthesizes NAAG from NAA and glutamate using ATP as energy source and is exclusively expressed in brain and spinal cord. To gain more information about the physiological function of BCG, we attempted to purify BCG hydrolase from mouse testis extracts and to identify it
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