Hen egg-white lysozyme [EC 3.2.1.17] was labelled with fluorescein-isothiocyanate (FITC). Three samples of FTC-lysozyme (S-A, S-B, and S-C) were prepared and the average amount of bound dye was 4 moles per mole of protein for S-A, 2 for S-B, and 1.5 for S-C. The enzymatic activity, circular dichromism (CD) and fluorescence characteristics of these conjugates were studied. The enzymatic activity of these labelled lysozymes on glycol chitin was almost the same as that of native lysozyme. No measurable change occurred in the 210–250 nm region of the CD spectra, whereas the CD spectra in the aromatic absorption region at 250–300 nm and the tryptophyl fluorescence spectra of FTC-lysozyme changed considerably with the degree of labelling, and it was suggested that at a high degree of labelling there might be some interaction between FTC groups and aromatic amino acid residues, probably tryptophyl residues, of the lysozyme molecule.
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Hiramatsu et al. (1973) studied this question.