Enhanced allosteric regulation of threonine deaminase and acetohydroxy acid synthase from Escherichia coli in a permeabilized-cell assay system.

A permeabilized-cell technique for rapid assay of enzyme activity has revealed enhanced allosteric regulation of both threonine deaminase (L-threonine hydrolyase (deaminating), EC 4.2.1.16) and acethohydroxy acid synthease (acetolactate pyruvate-lyase (carboxylating), EC 4.1.3.18) in Escherichia col K-12. In the permeabilized cell assay threonine deaminase exhibited a higher Hill coefficient for inhibition by L-isoleucine, and acetohydroxy acid synthase exhibited a hypersensensitivity to allosteric inhibition by L-valine when compared to studies on crude extracts. We propose that these effects reflect the in situ microenvironments of both enzymes. Preliminary evidence further indicates that acetohydroxy acid synthase may loosely associate with the cell membrane.