Reactions of Cyclic Amino Acid Derivatives with the Pyridoxal Phosphate of Cystathionase

Cystathionase, an enzyme requiring pyridoxal phosphate as a cofactor, reacts with several cyclic derivatives of α-aminobutyric acid to form complexes. The reaction depends on the presence of the pyridoxal phosphate moiety of the enzyme and is specific for the l form of the cyclic derivatives. Homocysteine thiolactone, α-aminobutyrolactone, α-aminobutyro-γ-selenolactone, and cycloserine competitively inhibit cystathionase activity. Conversely, the substrates homoserine and cystathionine competitively inhibit complex formation. An enzymic group with a pK of 8.6 participates in complex formation, whereas another more acidic group (pK 7.3) does not, but is ionized in the complex. This group apparently does not affect the absorption characteristics of the enzyme. These observations suggest a mechanism for transaldimination, based on a nucleophilic attack of the cyclic compounds on a protonated aldimine situated on the enzyme. Cystathionase normally has an absorption maximum at 420 mµ. When it was combined with α-aminobutyrolactone, homocysteine thiolactone, α-aminobutyro-γ-selenolactone, or cycloserine, the absorption maxima appeared in the range 495 mµ to 530 mµ. The shift to the longer wave lengths apparently corresponds to decreasing electronegativity of the heteroatoms in the cyclic derivatives. Stable enzyme-inhibitor intermediates are possible because of increased resonance energy in the complexes.