Catalytic reaction of the 2', 3'-dialdehyde analog of TPN (oTPN) with pig heart TPN-dependent isocitrate dehydrogenase in the presence of the substrate manganous isocitrate results in the formation of the dialdehyde derivative of TPNH (oTPNH). In the absence of the substrate, modification by oTPN leads to a progressive inactivation of the enzyme. The dependence of the pseudo-first order rate constants on the reagent concentration indicates the formation of a reversible complex with the enzyme prior to covalent modification (kmax = 5.5 X 10(-2) min-1; K1 = 290 microM). Reaction of [14C]oTPN with the enzyme results in the incorporation of 2 mol of oTPN/mol of peptide chain. No appreciable protection against either inactivation or incorporation by the natural ligands TPN and TPNH was obtained, suggesting different modes of binding of the analog in the presence and absence of the substrate isocitrate. Enzymatically synthesized oTPNH has been isolated and demonstrated to act as an affinity label for a TPNH-binding site of isocitrate dehydrogenase. The inactivation process exhibits saturation kinetics (kmax = 2.67 X 10(-3) min-1; K1 = 33 microM). Protection against activity loss, as well as a decrease in incorporation from 2 to 1 eq of [14C]oTPNH bound/peptide chain was observed in the presence of 1 mM TPNH. From the TPNH concentration dependence of the inactivation rate by oTPNH, a dissociation constant of 3.4 microM is calculated for TPNH, indicating binding of the analog to a specific TPNH-binding site on the enzyme. Although dialdehyde derivatives are frequently assumed to form Schiff bases with proteins, the evidence presented suggests the formation of morpholino derivatives as the products of the covalent reaction of isocitrate dehydrogenase with the dialdehyde derivatives of TPN and TPNH. The new reagent, oTPNH, may serve as an affinity label for other dehydrogenases.