title: Effect of guanidine hydrochloride on the holo- and apo-enzymes of pig heart lipoamide dehydrogenase.
abstract: 1. The effect of guanidine hydrochloride (GuHCl) on pig heart lipoamide dehydrogenase [NADH: lipoamide oxidoreductase, EC 1.6.4.3.] was investigated by means of enzymatic activity and optical measurements (CD, absorption, and fluorescence spectra). The activity of the enzyme decreased on increasing the concentration of GuHCl and the enzyme was completely inactivated in 2.0 M GuHCl. 2. The contents of alpha-helix, beta, and unordered forms in lipoamide dehydrogenase were estimated to be 34, 14, and 52%, respectively. On increasing the concentration of GuHCl, the content of alpha-helix in lipoamide dehydrogenase decreased, whereas the content of the beta form hardly changed. 3. The native lipoamide dehydrogenase showed absorption, CD, and fluorescence spectra characteristic of bound FAD in the visible region, suggesting hydrophobic interaction between the protein moiety and FAD chromophore. The absorption, CD, and fluorescence spectra of the enzyme in 2.0 M GuHCl were similar to those of free FAD in the buffer, suggesting the release of FAD from the protein moiety. 4. The protein fluorescence spectrum of lipoamide dehydrogenase had a maximum at 350 nm blue-shifted by 8 nm from that of tryptophan in aqueous solution. The maximum of the enzyme in 2.0 M GuHCl was red-shifted to 357 nm. This suggests exposure of tryptophan residues to a polar environment. The maximum, 352nm, of the apoenzyme shifted to 350 nm on addition of FAD. These results show that the conformation in the microenvironment of some tryptophan residues in lipoamide dehydrogenase is affected by the dissociation-association of FAD. 5. The contents of alpha-helix, beta, and unordered forms in the apoenzyme were estimated to be 35, 8, and 57%, respectively. These values are similar to those of the native holoenzyme. The alpha-helical structure in the apoenzyme molecule was more sensitive to GuHCl than that in the holoenzyme. FAD and two hydrophobic probes, 8-anilinonaphthalene-1-sulfonate (ANS) and 4 benzolamido-4'-aminostilbene-2,2'-disulfonate (MBAS), which can bind to the apoenzyme, stabilized the alpha-helical structure in the apoenzyme molecule.
mesh_terms: D000818:Animals; D001051:Apoenzymes / Q000378:metabolism; D001665:Binding Sites; D002942:Circular Dichroism; D008058:Dihydrolipoamide Dehydrogenase / Q000378:metabolism*; D006146:Guanidines / Q000494:pharmacology*; D007700:Kinetics; D046911:Macromolecular Substances; D009206:Myocardium / Q000201:enzymology*; D011485:Protein Binding; D011487:Protein Conformation; D013050:Spectrometry, Fluorescence; D013056:Spectrophotometry, Ultraviolet; D013552:Swine
chemical_list: D001051:Apoenzymes; D006146:Guanidines; D046911:Macromolecular Substances; D008058:Dihydrolipoamide Dehydrogenase