Reversible reaction of epsilon-amino groups of cytochrome c with salicylaldehyde to produce cytochrome c polymers

Slow-Onset, Potent Inhibition of Mandelate Racemase by 2-Formylphenylboronic Acid. An Unexpected Adduct Clasps the Catalytic Machinery

o-Carbonyl arylboronic acids such as 2-formylphenylboronic acid (2-FPBA) are employed in biocompatible conjugation reactions with the resulting iminoboronate adduct stabilized by an intramolecular N-B interaction. However, few studies have utilized these reagents as active site-directed enzyme inhibitors. We show that 2-FPBA is a potent reversible, slow-onset inhibitor of mandelate racemase (MR), an enzyme that has served as a valuable paradigm for understanding enzyme-catalyzed abstraction of an α-proton from a carbon acid substrate with a high pKa. Kinetic analysis of the progress curves for the slow onset of inhibition of wild-type MR using a two-step kinetic mechanism gave Ki and Ki* values of 5.1 ± 1.8 and 0.26 ± 0.08 μM, respectively. Hence, wild-type MR binds 2-FPBA with an affinity that exceeds that for the substrate by ∼3000-fold. K164R MR was inhibited by 2-FPBA, while K166R MR was not inhibited, indicating that Lys 166 was essential for inhibition. Unexpectedly, mass spectrometric analysis of the NaCNBH3-treated enzyme-inhibitor complex did not yield evidence of an iminoboronate adduct. 11B nuclear magnetic resonance spectroscopy of the MR·2-FPBA complex indicated that the boron atom was sp3-hybridized (δ 6.0), consistent with dative bond formation. Surprisingly, X-ray crystallography revealed the formation of an Nζ-B dative bond between Lys 166 and 2-FPBA with intramolecular cyclization to form a benzoxaborole, rather than the expected iminoboronate. Thus, when o-carbonyl arylboronic acid reagents are employed to modify proteins, the structure of the resulting product depends on the protein architecture at the site of modification.

Requirement of a reactive aldehyde moiety for aldehyde-mediated protection against cis-dichlorodiammineplatinum-induced cell inactivation

The effect of the aromatic aldehydes benzaldehyde and salicylaldehyde, the glucose-acetal derivative 4,6-benzylidene-D-glucose (BG) and the glucoside salicylaldehyde-beta-D-glucoside (helicin) on cell inactivation induced by cis-dichlorodiammineplatinum (cis-DDP) was investigated using cultured human NHIK 3025 cells. Cell inactivation was measured as loss in the ability of single cells to give rise to macroscopic colonies following drug treatment. The fraction of cells surviving a 2 hr treatment with 10 microM cis-DDP increased from 0.012 +/- 0.004 to 0.10 +/- 0.03 when treatment was combined with at least 1 mM benzaldehyde or at least 0.2 mM salicylaldehyde. Of the two sugar-aldehyde derivatives only helicin protected cells from the inactivating effect of cis-DDP, although to a much lesser extent than either benzaldehyde or salicylaldehyde. While helicin retains the aldehyde moiety of salicylaldehyde, BG does not possess any free aldehyde group. Using synchronized cells we found these effects to appear in all phases of the cell cycle. Measurements of cell-associated platinum indicated that the degree of protection from the inactivating effects of cis-DDP by these aldehydes was related to the degree of reduced platinum accumulation. We conclude that this reduced accumulation may represent an inhibition of specific cell membrane uptake sites via Schiff based formation between membrane amino groups and aldehydes.