An investigation of the properties of ornithine aminotransferase after inactivation by the 'suicide' inhibitor aminohexynoate and use of the compound as a probe of intracellullar protein turnover

Abstract

Ornithine aminotransferase is shown to bind 1 mol of amino[14C]hexynoate per mol of coenzyme in the 'suicide' inactivation process. At the same time the coenzyme pyridoxal phosphate becomes irreversibly bound to the enzyme protein. Apart from the inactivation, the labelled enzyme is indistinguishable from native ornithine aminotransferase by several separation techniques. Because the rate of degradation of the labelled enzyme is the same as that of the normal enzyme it is concluded that loss of coenzyme does not initiate turnover. Free aminohexynoate is rapidly eliminated from the liver, and 70% of the compound is excreted unchanged in 7.5 h. Inactivated ornithine aminotransferase accounts for 11% of the total labelled liver protein and significant amounts of label are found in aspartate aminotransferase which is also extensively inactivated. The rate of return of enzyme activity is determined and found to be more rapid than expected for a process in which the enzyme is synthesized at a constant rate and degraded in a single, first-order process.