CONFORMATION INVERSION OF BILIRUBIN FORMED BY REDUCTION OF THE BILIVERDIN HUMAN SERUM-ALBUMIN COMPLEX - EVIDENCE FROM CIRCULAR-DICHROISM

As shown by circular dichroism spectroscopy, biliverdin preferentially adopts an M-helicity conformation on human serum albumin in aqueous buffer, pH 7.5, whereas biliverdin exhibits only a weak preference for the P-helicity conformation on bovine serum albumin at the same pH. Upon rapid reduction of the complexes with sodium borohydride, P-helicity bilirubin-IXα is obtained on the human albumin complex, and M-helicity bilirubin-IXα is obtained on the bovine serum albumin complex. Thus, biliverdin in effect undergoes an inversion of chirality upon reduction. Since the reduction did not afford a rubin with the same helicity as that of the verdin, the observations point to a hitherto undetected conformational mobility of albumin-bound bilirubin. © 1992.