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 modifies heme catalytic reactivity, the effects of Hx, human serum Alb (HSA), and bovine serum Alb (BSA) on the peroxidase- and catalaselike activities of hemin were investigated. These hemin activities were found to be inhibited by 50 to 60% with either HSA or BSA, and by 80 to 90% with Hx. The heme complexes with Hx or Alb (1:1 = protein:heme) therefore had a much lower reactivity toward H2O2 and Cum-OOH than the nonprotein heme. A kinetic analysis suggested that binding to Hx or Alb inhibited the primary activation of heme by H2O2, the step common for both peroxidase- and catalaselike activities of hemin. It is thought that by complexing heme, the Hx and Alb can prevent the toxic effects of extracellular heme in blood plasma.