Circular Dichroism of Human Lysozyme<xref ref-type="fn" rid="fn1"><sup>*</sup></xref>

Abstract

Effects of pH and the various inhibitors on the circular dichroic (CD) spectrum of human urine lysozyme were studied and compared with those for hen egg-white lysozyme [EC 3. 2. 1. 17]. Human lysozyme gave tryptophyl CD maxima at 305 and 293.5 mμ with a negative and positive ellipticity, respectively, at neutral pH values. On lowering the pH, the both CD maxima changed in a manner similar to that for the corresponding maxima at 305 and 295 mμ of hen egg-white lysozyme. This change was thus ascribed to the change in the interaction of Trp-108 and Glu-35. At alkaline pH values, as the tyrosyl residues were ionized, a conformation-dependent CD band with a large negative ellipticity newly appeared at about 315 mμ. Since this wavelength was much longer than that for the corresponding CD band (298 mμ) of hen egg-white lysozyme, this CD band reflects a special interaction between an ionized tyrosyl and other residues. Effects of di- and tri-N-acetylglucosamine on the CD band at 305 mμ were very similar to those for hen egg-white lysozyme. Both the equilibrium mixture of N-acetylglucosamine (NAG) and its β-methyl glycoside, however, gave no significant effect on the ellipticity at this wavelength; this fact differed from that for hen egg-white lysozyme. All the inhibitors studied also slightly enhanced another tryptophyl CD maximum at 293.5 mμ; this was a contrast to a large enhancement of the corresponding maximum at 295 mμ of hen egg-white lysozyme. All these inhibitors also reduced the negative ellipticity at about 275 mμ. The extent of this reduction by di- and tri-NAG was greater than that for NAG and its β-methyl glycoside. The CD changes produced by tri-NAG at alkaline pH values were found to be very similar to those for the binding at neutral pH values.