Cofactors-i.e., metal ions and coenzymes-extend the catalytic scope of enzymes and might have been among the first biological catalysts. They may be expected to efficiently extend the catalytic potential of antibodies. Monoclonal antibodies (MAbs) against N-alpha-phosphopyridoxyl-L-lysine were screened for 1) binding of 5'-phosphopyridoxyl amino acids, 2) binding of the planar Schiff base of pyridoxal-5'-phosphate (PLP) and amino acids, the first intermediate of all PLP-dependent reactions, and 3) catalysis of the PLP-dependent alpha, beta-elimination reaction with beta-chloro-D/L-alanine. Antibody 15A9 fulfilled all criteria and was also found to catalyze the cofactor-dependent transamination reaction of hydrophobic D-amino acids and oxo acids (k'(cat) = 0.42 min(-1) with D-alanine at 25 degrees C). No other reactions with either D- or L-amino acids were detected. PLP markedly contributes to catalytic efficacy-it is a 10(4) times more efficient acceptor of the amino group than pyruvate. The antibody ensures reaction specificity, stereospecificity, and substrate specificity, and further accelerates the transamination reaction (k'(cat(Ab))/k'(cat(PLP)) = 5 x 10(3)). The successive screening steps simulate the selection criteria that might have been operative in the evolution of protein-assisted pyridoxal catalysis.