title: Heme binding to human alpha-1 proteinase inhibitor.
abstract: BACKGROUND
  Heme is a unique prosthetic group of various hemoproteins that perform diverse biological functions; however, in its free form heme is intrinsically toxic in vivo. Due to its potential toxicity, heme binding to plasma proteins is an important safety issue in regard to protein therapeutics derived from human blood. While heme binding by hemopexin, albumin and α(1)-microglobulin has been extensively studied, the role of other plasma proteins remains largely unknown.
  
  
  METHODS
  We examined two acute-phase plasma proteins, haptoglobin (Hp) and alpha-1 proteinase inhibitor (α(1)-PI) for possible interactions with heme and bilirubin (BR), the final product of heme degradation, using various techniques: UV/Vis spectroscopy, fluorescence, circular dichroism (CD), and surface plasmon resonance (SPR).
  
  
  RESULTS
  According to our data, Hp exhibits a very weak association with both heme and BR; α(1)-PI's affinity to BR is also very low. However, α(1)-PI's affinity to heme (K(D) 2.0×10(-8)M) is of the same order of magnitude as that of albumin (1.26×10(-8)M). The data for α(1)-PI binding with protoporphyrin IX (PPIX) suggest that the elimination of the iron atom from the porphyrin structure results in almost 350-fold lower affinity (K(D) 6.93×10(-6)M), thus indicating that iron is essential for the heme coordination with the α(1)-PI.
  
  
  CONCLUSIONS
  This work demonstrates for the first time that human α(1)-PI is a heme binding protein with an affinity to heme comparable to that of albumin.
  
  
  CONCLUSIONS
  Our data may have important implications for safety and efficacy of plasma protein therapeutics.
authors.affiliation: Center for Biologics Evaluation and Research, Food and Drug Administration, Bethesda, MD 20892, USA. elena.karnaukhova@fda.hhs.gov
authors.forename: Elena
authors.initials: E
authors.lastname: Karnaukhova
authors.forename: Sonia Silinsky
authors.initials: SS
authors.lastname: Krupnikova
authors.forename: Mohsen
authors.initials: M
authors.lastname: Rajabi
authors.forename: Abdu I
authors.initials: AI
authors.lastname: Alayash
mesh_terms: D001663:Bilirubin / Q000737:chemistry / Q000378:metabolism*; D002352:Carrier Proteins / Q000737:chemistry / Q000378:metabolism*; D002942:Circular Dichroism; D006242:Haptoglobins / Q000737:chemistry / Q000378:metabolism*; D006418:Heme / Q000737:chemistry / Q000378:metabolism*; D000081062:Heme-Binding Proteins; D006420:Hemeproteins / Q000737:chemistry / Q000378:metabolism*; D006801:Humans; D011487:Protein Conformation; D011524:Protoporphyrins / Q000737:chemistry / Q000378:metabolism*; D013056:Spectrophotometry, Ultraviolet; D020349:Surface Plasmon Resonance; D014361:Trypsin Inhibitors / Q000737:chemistry / Q000378:metabolism; D000515:alpha 1-Antitrypsin / Q000737:chemistry / Q000378:metabolism*
chemical_list: D002352:Carrier Proteins; D006242:Haptoglobins; D000081062:Heme-Binding Proteins; D006420:Hemeproteins; D011524:Protoporphyrins; D014361:Trypsin Inhibitors; D000515:alpha 1-Antitrypsin; D006418:Heme; C028025:protoporphyrin IX; D001663:Bilirubin