Nonequivalent binding sites in cystathionase. Nanosecond and steady fluorescence studies

The analogs P-pyridoxyl-L-alanine and P-pyridoxyl-L-homoserine bind to the apoprotein of the enzyme cystathionase and inhibit the reactivation of enzymatic activity after addition of pyridoxyl-5-P. The binding of the inhibitors was monitored by measuring the fluorescence emitted by the P-pyridoxyl moiety at 395 nm (excitation 325 nm). The fluorometric titration results indicate the presence of nonequivalent binding sites in the apoprotein. A model based on two classes of independent binding sites fits the fluorometric data reasonably well. The presence of nonequivalent fluorescent sites in reduced cystathionase was also detected by nanosecond spectroscopy. In contrast to the model compound P-pyridoxyl-epsilon-lysine (tau equals 2.6 ns), the P-pyridoxyl residues of cystathionase display multiexponential fluorescence decay. Two fluorescence lifetimes (tau2 equals 4.1 ns and tau2 equals 15 ns) fit the deconvoluted decay results obtained by pulse fluorimetry. It is proposed that the P-pyridoxyl chromophores of reduced cystathionase have different environments.