Spectral evidence for a flavin adduct in a monoalkylated derivative of pig heart lipoamide dehydrogenase.

A derivative of the flavoprotein pig heart lipoamide dehydrogenase has been described recently (Thorpe, C., and Williams, C.H. (1976) J. Biol. Chem. 251, 3553-3557), in which 1 of the 2 cysteine residues generated on reduction of the intrachain active center disulfide bridge is selectively alkylated with iodoacetamide. This monolabeled enzyme exhibits a spectrum of oxidized bound flavin. The addition of 1 mM NAD+ to this derivative at pH 8.3 causes a decrease in absorbance of approximately 50% at 448 nm, with a concomitant increase at 380 nm. These spectral changes are complete within 3 ms and are reversible. NAD+ titrations generate isosbestic points at 408, 374, and 327 nm; allowing values for the apparent dissociation constant for NAD+ and the extent of bleaching at infinite ligand to be obtained from double reciprocal plots. Between pH 6.1 and 8.8, the apparent KD decreases from 320 to 35 muM, whereas the extrapolated delta epsilon 448 values remain approximately constant at 1/2 epsilon 448. Direct measurement of NAD+ binding by gel filtration at pH 8.8 indicates that the spectral changes are associated with a stoichiometry of 1.2 mol of NAD+ bound/2 mol of FAD. The modified protein is a dimer containing 1 FAD and 1 alkylated cysteine residue/subunit; the native enzyme is also dimeric. The visible spectrum of the species absorbing at 380 nm, approximated by correction for the residual oxidized FAD, shows a single maximum at 384 nm, epsilon 384 = 8.7 mM-1cm-1. Comparison of this spectrum with that of model compounds of known structure suggests that it may represent a reversible covalent flavin adduct induced on binding NAD+.