Effect of sodium dodecyl sulfate on circular dichroism and enzyme activities of carboxypeptidases A and B

Abstract

Conformational transitions in dilute solutions of bovine carboxypeptidases A and B and porcine carboxypeptidase B were incited by sodium dodecyl sulfate, and these transitions were tested by CD measurement in the 200–320-nm spectral zone. At ratios of 10–100 moles sodium dodecyl sulfate to 1 mole enzyme (pH 7.5 at room temperature) the CD bands at 250–320 nm progressively diminished whereas the CD bands at 210–225 nm were affected slightly. The decrease of [θ] 294 nm of carboxypeptidase A showed biphasic first-order kinetics in the detergent to enzyme ratios tested, and the peptidase and esterase activities decreased in a similar manner. The near ultraviolet CD spectrum of bovine and porcine carboxypeptidases B had several relatively strong positive CD bands at 250–280 nm and one negative band at 292 nm. The decrease in magnitude of [θ] 292 nm with increased amount of sodium dodecyl sulfate and incubation time was biphasic and the peptidase activity decreased in a similar way. However, the esterase activity of both carboxypeptidases B was slightly enhanced on interaction with low concentration of sodium dodecyl sulfate. The negative conformation-dependent CD bands at 210–225 nm of the carboxypeptidases B were more prominent than those of carboxypeptidase A. The effect of sodium dodecyl sulfate on the enzyme side chains, as expressed by the changes of the bands at 286–294 nm, suggests the involvement of tryptophan residues in these interactions. The CD data also indicate that the conformation of carboxypeptidases B differs from that of carboxypeptidase A, as can be expected from the differences in their amino acid composition. The effect of sodium dodecyl sulfate, however, is restricted chiefly to the enhancement of rotational freedom of the side chain chromophores, and the backbone conformation is affected only by very high concentrations of sodium dodecyl sulfate.