Circular dichroism of κ- and λ-type Bence-Jones proteins at various pH values and temperatures

Abstract

Comparison of the near ultraviolet circular dichroism (CD) spectra of κ- and λ-type Bence-Jones proteins showed differences which, in some cases, were correlated with the proteins' antigenic type. The far ultraviolet (190–250 nm) CD spectra showed that both antigenic types had a negative band at 216–218 nm indicative of β structure. A consistent finding on 5 κ and 7 λ samples was that the positive CD band near 200 nm was resolved into a single band (200–204 nm) for the κ type proteins and into two positive bands (at 190–195 nm and at 197–204 nm) for the λ-type proteins. The positive ellipticity near 200 nm was progressively decreased as the pH was lowered and was replaced by a negative trough at pH 2. Increasing the temperature (25–60 °C) also caused a progressive decrease in the positive ellipticity near 200 nm. The results indicated that acid and heating to 60 °C produced an unfolding of the native structure which was partially reversible. CD spectra showed that heating to 95 °C resulted in irreversible conformational changes which led to an increase in the amount of random chain structure. Physical and immunological characterization of one sample indicated that it was a half ( M r 11 500) of a λ-type Bence-Jones protein corresponding to the constant portion. The changes in the far ultraviolet CD spectrum at various temperatures for this sample were similar to those obtained with intact Bence-Jones proteins. CD studies between 25 and 60 °C were also performed on a κ-type Bence-Jones protein corresponding to the variable half, and it was found that the degree of unfolding was greater for the variable half than for the constant half.