Binding of heparin to human antithrombin III activates selective chemical modification at lysine 236. Lys-107, Lys-125, and Lys-136 are situated within the heparin-binding site of antithrombin III.

A new water-soluble color reagent, 4-N,N-dimethylaminoazobenzene-4'-isothiocyano-2'-sulfonic acid (S-DABITC), was used to identify lysine residues of antithrombin III which participate in the binding of heparin. Antithrombin, modified with S-DABITC in the presence and absence of low molecular weight heparin (Mr 5000) was reduced, carboxymethylated, and digested with trypsin. The digest was analyzed by high-performance liquid chromatography and monitored at 465 nm. In the absence of heparin, four major colored peptides (T1, T2, T3, and T4) were identified. When antithrombin was preincubated with heparin (2-fold by weight), followed by S-DABITC modification, the recovery of peptide T4 remained unchanged, but the recoveries of T1, T2, and T3 were reduced by 93, 86, and 98%, respectively. In addition, a new colored peptide, TA, appeared. Amino acid sequencing of peptides T1, T2, T3, and TA localized S-DABITC modification sites as Lys-136, Lys-125, Lys-107, and Lys-236, respectively. Thus, binding of heparin to human antithrombin diminished S-DABITC modification at Lys-107, Lys-125, and Lys-136, but at the same time enhanced S-DABITC modification at Lys-236. This phenomenon was further characterized by varying the molar ratio of heparin/antithrombin (from 0.04 to 20). The shielding of Lys-125 and Lys-136 was inversely proportional to the activation of Lys-236. At a heparin/antithrombin molar ratio of 1, the extent of shielding of Lys-125 and Lys-136 and the unmasking of Lys-236 were 25-33%. This shielding-unmasking effect correlated with enhanced antithrombin inhibition of thrombin. We conclude that Lys-107, Lys-125, and Lys-136 are situated within the heparin-binding site of human antithrombin and that binding of heparin to antithrombin causes a conformational change of antithrombin that leads to the exposure of Lys-236 for S-DABITC modification.