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J. Biol. Chem., Vol. 277, Issue 22, 19720-19726, May 31, 2002

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Clustered Charged Amino Acids of Human Adenosine Deaminase Comprise a Functional Epitope for Binding the Adenosine Deaminase Complexing Protein CD26/Dipeptidyl Peptidase IV^*

Eva Richard^§, S. Munir Alam, Francisco X. Arredondo-Vega, Dhavalkumar D. Patel^¶, and Michael S. Hershfield^**

From the Departments of  Medicine, ^¶ Immunology, and  Biochemistry, Duke University Medical Center, Durham, North Carolina 27710

Human adenosine deaminase (ADA) occurs as a 41-kDa soluble monomer in all cells. On epithelia and lymphoid cells of humans, but not mice, ADA also occurs bound to the membrane glycoprotein CD26/dipeptidyl peptidase IV. This "ecto-ADA" has been postulated to regulate extracellular Ado levels, and also the function of CD26 as a co-stimulator of activated T cells. The CD26-binding site of human ADA has been localized by homolog scanning to the peripheral 2-helix (amino acids 126-143). Among the 5 non-conserved residues within this segment, Arg-142 in human and Gln-142 in mouse ADA largely determined the capacity for stable binding to CD26 (Richard, E., Arredondo-Vega, F. X., Santisteban, I., Kelly, S. J., Patel, D. D., and Hershfield, M. S. (2000) J. Exp. Med. 192, 1223-1235). We have now mutagenized conserved 2-helix residues in human and mouse ADA and used surface plasmon resonance to evaluate binding kinetics to immobilized rabbit CD26. In addition to Arg-142, we found that Glu-139 and Asp-143 of human ADA are also important for CD26 binding. Mutating these residues to alanine increased dissociation rates 6-11-fold and the apparent dissociation constant K_D for wild type human ADA from 17 to 112-160 nM, changing binding free energy by 1.1-1.3 kcal/mol. This cluster of 3 charged residues appears to be a "functional epitope" that accounts for about half of the difference between human and mouse ADA in free energy of binding to CD26.

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