Plasma haptoglobin in cattle (Bos taurus) exists as polymers in association with albumin

1. Plasma haptoglobin (Hp) in cattle (Bos taurus) has a molecular mass so large that it is virtually unable to penetrate 4% polyacrylamide gels and is excluded from gel filtration media with an upper exclusion limit of greater than 1000 kDa. 2. In most species, apart from ruminants, Hp has a molecular mass of 100 kDa, consisting of two subunits of 40 kDa and two subunits of 9 kDa, although in a few species, such as man, a genetic variant of Hp forms polymers of higher mass. 3. Bovine Hp was purified by (i) salt precipitation, gel filtration and ion exchange chromatography and (ii) by affinity chromatography and gel filtration, with final purification by (iii) preparative electrophoresis. 4. Bovine Hp could only penetrate sodium dodecyl sulphate polyacrylamide gels following denaturation with mercaptoethanol, when it was shown to be composed of subunits of 40 and 16 kDa, but a protein of 67 kDa was consistently present. 5. Western blotting identified the 67 kDa protein as bovine serum albumin. 6. The polymeric form of Hp found in bovine plasma is formed by association of 40 and 16 kDa subunits with albumin.