Dehydrogenase binding by tiazofurin anabolites

Area of Science:

• Biochemistry
• Pharmacology
• Enzymology

Background:

• Tiazofurin is a novel antitumor agent.
• Thiazole-4-carboxamide adenine dinucleotide (TAD) is its active metabolite.
• TAD is structurally similar to NAD+, with a thiazole-4-carboxamide replacing nicotinamide.

Purpose of the Study:

• To investigate the inhibitory mechanism of TAD on various mammalian dehydrogenases.
• To compare TAD's binding affinity to its target enzyme, IMP dehydrogenase (IMPd), versus other dehydrogenases.
• To explore the structural basis for TAD's enzyme interactions using computer modeling.

Main Methods:

• Enzyme inhibition kinetics assays were performed for TAD against NAD+-dependent dehydrogenases (glutamate, alcohol, lactate, malate).
• Binding affinities of TAD to IMPd and other dehydrogenases were compared.
• Computer modeling was used to analyze the binding interactions of TAD with dehydrogenase cofactor sites.

Main Results:

• TAD competitively inhibits mammalian glutamate, alcohol, lactate, and malate dehydrogenases with respect to NAD+.
• TAD binds to these dehydrogenases with 10-100 times lower affinity than to IMPd.
• Computer modeling suggests TAD binds to the cofactor site, mimicking nicotinamide's properties, but weaker binding to dehydrogenases may stem from an inability to maintain specific sulfur-oxygen contacts.

Conclusions:

• TAD exhibits inhibitory activity against a range of NAD+-dependent dehydrogenases, in addition to its known target IMPd.
• The differential binding affinities suggest a potential basis for the selectivity of TAD's action.
• Understanding these interactions is crucial for optimizing tiazofurin's therapeutic potential as an antitumor agent.