Schematic representation of membrane-bound factor Va/Va^Leiden heavy-chain inactivation by APC. Normal plasma factor Va is inactivated following three ordered and sequential cleavages in the heavy chain at Arg⁵⁰⁶, Arg³⁰⁶, and Arg⁶⁷⁹.45 Cleavage at Arg⁵⁰⁶, which gives rise to an M_r = 75,000 fragment and an M_r = 28/26,000 doublet, is necessary to optimally expose the site at Arg³⁰⁶. Further cleavage at Arg³⁰⁶yields an M_r = 45,000 fragment and an M_r = 30,000 fragment.45 Individuals with the Arg⁵⁰⁶→ Gln mutation (factor V^Leiden) no longer have a cleavage site at position 506, which slows the rate of cleavage at Arg³⁰⁶.46 Cleavage at Arg³⁰⁶ yields an M_r = 45,000 fragment and an M_r = 60/58,000 doublet. Further cleavage of the M_r = 60/58,000 doublet at Arg⁶⁷⁹ yields an Mr = 54,000 fragment. Fragments that are recognized by the monoclonal antibody (HFVa_HC#17) used in this study are indicated by the shaded boxes.25