Optical activity of disulfide bonds in proteins: studies on human choriomammotropin and bovine pituitary somatotropin.

The contributions of the homologous carboxyl-terminal disulfide bonds in human choriomammotropin (human chorionic somatomammotropin, human placental lactogen) and bovine somatotropin (pituitary growth hormone), to the near ultraviolet circular dichroism spectra of these two proteins have been evaluated. The disulfide bond in the human placental protein displays a broad, negative band centered near 260 nm ([theta]M, 260nm = -2100 +/- 160 deg cm2 dmol-1) which is equivalent, within experimental error, to the band previously assigned to the identical disulfide in plasmin modified human somatotropin. The homologous disulfide in the bovine hormone also exhibits a negative band, very similar in intensity ([theta]M, 254nm = -2200 +/- 210 deg cm2 dmol-1), with an estimated band center blue-shifted relative to the human proteins to 252-255 nm. Reoxidation of either partially reduced protein results in complete repair of the circular dichroism spectrum to that of the native protein. No definite contributions could be assigned below 240 nm to the optical activity of these disulfide bonds. Circular dichroism measurements have also been used to approximate the rates of reduction of the two proteins.