Activated protein C (APC) is useful in the treatment of sepsis. Ischemia and acidosis, which often accompany sepsis, cause the release of copper from loosely bound sites. We investigated (i) whether physiological concentrations of copper inhibit APC anticoagulant activity and (ii) if any copper-induced A-PC inhibition is reversible by human serum albumin (HSA) or a high-affinity copper-binding analogue of the human albumin N-terminus, d-Asp-d-Ala-d-His-d-Lys (dDAHK). APC activity after 30 min of incubation with Cucl(2) (10 muM) was decreased 26% below baseline. HSA, both alone and when combined with various ratios of CuCl2 increased APC activity significantly above baseline. d-DAHK alone and 2:1 and 4:1 ratios of dDAHK:CuCl2 also increased APC activity. A-PC contained 1.4 muM copper, which helps explain the increased APC activity with HSA and d-DAHK alone. These in vitro results indicate that copper inhibits A-PC activity and that albumin and d-DAHK reverse the copper-induced APC deactivation. (C) 2002 Elsevier Science (USA).