Some serum proteins have a finite capacity for binding hemoglobin and its degradation products such as hematin and bilirubin. Of these proteins, the haptoglobins alone bind free circulating hemoglobin,1,2 a globulin and albumin bind hematin,1,3-7 while only albumin binds bilirubin.8 The globulin has been called the heme binding globulin, since it has a greater affinity for hematin than does albumin.3 This heme binding protein is identified by electrophoresis as a fast α2-globulin on starch gel,1 as a β-globulin on paper,3,7,14 and as α3− and β-globulins on cellulose acetate 9 and is therefore distinctly different from the haptoglobins.
The heme binding protein and the haptoglobins also differ in their in vivo life cycles. In chronic hemolytic processes, the haptoglobins will bind hemoglobin and disappear from the plasma before the heme binding protein becomes saturated and is removed from the circulation.1,7 On