Binding of bilirubin by bovine and human alpha-fetoprotein.

alpha-Fetoprotein, a fetal protein associated with certain tumors, was found to bind bilirubin. Addition of human or bovine alpha-fetoprotein to bilirubin solutions enhanced the light absorbance of bilirubin and shifted its maximum. Bovine alpha-fetoprotein caused a marked shift towards shorter wavelengths, while human alpha-fetoprotein gave a slight red shift. The spectral changes were used to study the characteristics of the binding of bilirubin by bovine alpha-fetoprotein. These studies indicated the presence of one binding site/molecule of alpha-fetoprotein with an association constant of about 1.1 . 10(6) M-1. A difference between the spectral changes brought about by alpha-fetoprotein and albumin allowed comparison of their relative affinities for bilirubin. The spectrum approximated the average between the spectra induced by the two proteins when the ratio of bovine alpha-fetoprotein to bovine albumin was 6.3 : 1, and of the human proteins 21 : 1, respectively. These results show that alpha-fetoprotein from two species binds bilirubin with an affinity somewhat lower than that of albumin. Binding of bilirubin by alpha-fetoprotein is in agreement with the recent demonstration of structural homology between alpha-fetoprotein and albumin. Whether alpha-fetoprotein plays a role in the metabolism of bilirubin or other degradation products of heme remains to be investigated.     

Ligands induce conformational changes in the carboxyl-terminus of progesterone receptors which are detected by a site-directed antipeptide monoclonal antibody.

We have prepared a monoclonal antibody, C-262, to a synthetic peptide that contains the carboxy-terminal 14 amino acids from progesterone receptors (PR). This sequence is 100% conserved in all species of PRs that have been cloned to date, suggesting that this antibody will recognize all mammalian and avian PR. The C-262 antibody recognizes both native and denatured forms of the receptor. However, it does not recognize PR when they are bound to the hormone agonists progesterone or R5020. Surprisingly the antibody does recognize PR when they are bound to the steroid antagonist RU486. This suggests that progestin agonists induce a conformational change in the receptor that occludes the C-262 epitope in the carboxyl-terminus, whereas unliganded receptors and receptors bound with RU486 assume distinct conformations that leaves the C-terminal tail accessible to the C-262 antibody.     

Monoclonal antibodies to the bovine immunoglobulin G2a allotypes A1 and A2.

Production of murine monoclonal antibodies (Mabs) to bovine IgG2a-A1 and A2 allotypes resulted in three Mabs being selected as anti-IgG2a-A1 and A2 reagents. Two Mabs recognize the A1 allotype and 1 recognizes the A2 allotype. Initial epitope mapping with the Mabs indicates that one of the A1 epitopes resides in the hinge region and the other epitope resides more toward the C-terminus of the immunoglobulin. The A2 epitope recognized by the A2 Mab does not appear to reside in the hinge region of the immunoglobulin but apparently resides more C-terminal.