| dc.description.abstract | Ethionamide (2-ethyl, 4-thioisonicotinamido) is an active tuberculostatic second-line drug, which plays an important role in the treatment of pulmonary tuberculosis resistant to one or more of the classical drugs. The drug possesses very high activity and relatively low toxicity, thus offering good scope for the treatment of tuberculosis, especially in cases of infection caused by organisms resistant to the primary drugs, viz., INH or SM.
The in vivo activity of ethionamide in experimentally infected animals is considerably greater than its in vitro activity, which indicates that the drug exercises its action through its metabolites. In the absence of any detailed systematic study of the metabolism of ethionamide, work was undertaken to elucidate this aspect. Some of the metabolites of ethionamide after its oral administration to guinea pigs have been isolated and characterized. These and a few other possible metabolites and derivatives of ethionamide have been chemically synthesized and their antituberculous activities in vitro have been examined. Ethionamide sulphoxide has been shown to be the major metabolite; furthermore, ethionamide sulphoxide inhibits the growth of the organism Mycobacterium tuberculosis H37Rv (in vitro) more effectively than ethionamide itself.
An enzyme catalyzing the conversion of ethionamide to its sulphoxide in the presence of NADPH and oxygen has been detected in various tissues of guinea pig. The maximum activity was obtained in liver homogenates associated with the microsomal fraction. The enzyme has been purified from guinea pig liver homogenates and has been shown to be a monooxygenase. The purified enzyme is homogeneous by antigenic, ultracentrifugal, and electrophoretic criteria. It has a molecular weight of 85,000 (an average of three methods) and has been shown to contain 1 gm atom of iron and 1 mole of FAD per mole. The enzyme requires stoichiometric amounts of oxygen and NADPH for the sulphoxidation reaction. The molecular activity (number of moles of substrate converted per min per mole of enzyme under optimal conditions of pH and temperature) has been calculated to be 2.1.
The biochemical properties of the enzyme, such as optimal pH and temperature for activity, sensitivity to various inhibitors, substrate specificity, stability, and the Km values for the substrate, have been studied. The mechanism of sulphoxidation has been examined. The enzyme superoxide dismutase prepared from ovine erythrocytes inhibits the sulphoxidation reaction in the enzymatic system, indicating the involvement of superoxide anion in the reaction. In addition, the sulphoxidation of ethionamide has also been demonstrated in a non-enzymatic model system involving phenazine methosulphate-NADH-O , known to generate superoxide anions. Even this system is sensitive to the action of superoxide dismutase. Thus the involvement of superoxide anions in yet another class of reactions, viz., sulphoxidation, has been established.
The enzyme is inducible on administration of the drug, and the maximal levels of enzyme activity are reached between 12-24 hours after the administration of the drug; the levels of enzyme activity go down in 36 hours and reach normal levels by 48 hours. Although the enzyme catalyzes the conversion of a variety of substrates containing C=S grouping to their sulphoxides, it is specifically induced by ethionamide. The induction is due to the increased synthesis of the enzyme.
This enzyme, distributed in various tissues, has also been purified. A comparison of the properties of the enzymes derived from various tissues reveals that it is identical in all aspects studied. | |
