Substrate specificity of prostate-specific membrane antigen.

A series of putative dipeptide substrates of prostate-specific membrane antigen (PSMA) was prepared that explored alpha- and beta/gamma-linked acidic residues at the P1 position and various chromophores at the P2 position, while keeping the P1' residue constant as L-Glu. Four chromophores were examined, including 4-phenylazobenzoyl, 1-pyrenebutyryl, 9-anthracenylcarboxyl-gamma-aminobutyryl, and 4-nitrophenylbutyryl. When evaluating these chromophores, it was found that a substrate containing 4-phenylazobenzoyl at the P2 position was consumed most efficiently. Substitution at the P1 position with acidic residues showed that only gamma-linked L-Glu and D-Glu were recognized by the enzyme, with the former being more readily proteolyzed. Lastly, binding modes of endogenous substrates and our best synthetic substrate (4-phenylazobenzoyl-Glu-gamma-Glu) were proposed by computational docking studies into an X-ray crystal structure of the PSMA extracellular domain.