Hemopexin and Albumin Metabolism in Porphyria Cutanea Tarda

Serum contains two major proteins, hemopexin (Hx) and albumin (Alb), which bind porphyrins and metalloporphyrins. The heme-binding property of the 3-glycoprotein, Hx, was recognized by Schultze et al. in 1961 (9). Heme is specifically bound by Hx with a Kd of < 10-12 M (3) and also by the more abundant protein, Alb, but with a Kd of 10-8 M. Both proteins display the same affinity for uro- and protoporphyrin (Kd 10-6 M); whereas Hx binds coproporphyrin (Kd 10-7 M) better than Alb (Kd 10-6 M (3). The higher affinity of Hx for coproporphyrin suggests that Hx may be instrumental in the disposal of this porphyrin similar to its role in binding heme in hemolytic diseases (11). The present study was initiated to assess whether the accumulation of porphyrins in porphyria cutanea tarda (PCT) mediated by estrogen and/or alcohol, would enhance the catabolism of Hx and/or Alb since we had observed a decreased serum level of both proteins in patients with this condition (6).