Most of the known cellular substrates of the ubiquitin system are short-lived growth regulators and transcriptional activators. Very few enzymes involved in intermediary metabolism have been shown to be targeted by the system. In a reconstituted cell-free system, we show that tyrosine aminotransferase (TAT), a key enzyme involved in amino acid metabolism, is conjugated and degraded in an ATP- and ubiquitin-dependent manner. Degradation of ubiquitin-TAT adducts requires, in addition to the 26S proteasome, a novel, yet unidentified, factor. TAT can be protected from degradation by association with its coenzyme pyridoxal phosphate. To examine the potential role of the ubiquitin system in regulating the stability of the enzyme in vivo, we show that cell extracts derived from livers of animals in which TAT was induced, display a corollary increase in the formation of specific TAT-ubiquitin adducts.