Optical activity of disulfide bonds in proteins. 1. Studies on plasmin modified human somatotropin

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Abstract
Reduction and alkylation of the 2 disulfide bonds in a preparation of human pituitary growth hormone which were previously modified by limited proteolysis with the enzyme plasmin were studied. Quantitative and selective reduction of the carboxyl-terminal disulfide and total reduction of both disulfides, was achieved in the absence of denaturants. Circular dichroism spectra of the various reduced and reduced-alkylated derivatives have provided sufficient information to allow an estimation of the individual contributions of each disulfide bond to the total optical activity of the protein. These contributions represented a significant portion of the total optical activity between 290-250 nm. The carboxyl-terminal bond exhibits negative dichroism with an apparent center near 258 nm ([.theta.]M,258nm = 2100.degree. cm2 dmol-1). The contribution of the remaining disulfide is red-shifted to 293 nm, is also negative in sign, and somewhat more intense ([.theta.]M,273nm = -3200.degree. cm2 dmol-1). Circular dichroism measurements were also used to approximate the rate of reduction of the protein.