Interaction of bovine glutamate dehydrogenase with isoquinoline alkaloids.

Representatives of four different classes of isoquinoline alkaloids (aporphines, benzylisoquinolines, tetrahydroisoquinolines, and phthalideisoquinolines) inhibit glutamate dehydrogenase activity in both reaction directions. Representatives of five other groups of isoquinoline alkaloids have been without any significant effect on this enzyme. The structural requirements for the interaction of alkaloids with glutamate dehydrogenase and alcohol dehydrogenase are different. From the analysis of inhibition and from fluorescence-polarization data it follows that for an efficient binding of these alkaloids to glutamate dehydrogenase the presence of coenzyme is necessary. The effects of substrates on the interaction of the enzyme with alkaloids are small. Experiments with other ligands of glutamate dehydrogenase have been done to locate the binding site of the enzyme for alkaloids. The observed kinetic competition of isoquinoline alkaloids with thyroxine might indicate common or overlapping binding sites of glutamate dehydrogenase for these compounds.