The effects of heme-binding proteins on the peroxidative and catalatic activities of hemin

The plasma proteins hemopexin (Hx) and albumin (Alb) are known to bind heme with high and medium affinity, respectively. To study how this binding mod ifies heme catalytic reactivity, the effects of Hx, human serum Alb (HSA), and bovine serum Alb (BSA) on the peroxidase- and catalaselike activities o f hemin were investigated. These hemin activities were found to be inhibite d by 50 to 60% with either HSA or BSA, and by 80 to 90% with Hx. The heme c omplexes with Hx or Alb (1:1 = protein:heme) therefore had a much lower rea ctivity toward H2O2 and Cum-OOH than the nonprotein heme. A kinetic analysi s suggested that binding to Hx or Alb inhibited the primary activation of h eme by H2O2, the step common for both peroxidase- and catalaselike activiti es of hemin. It is thought that by complexing heme, the Hx and Alb can prev ent the toxic effects of extracellular heme in blood plasma. (C) 1999 Elsev ier Science Inc.