RECOMBINANT HUMAN PHENYLALANINE-HYDROXYLASE IS A SUBSTRATE FOR THE UBIQUITIN-CONJUGATING ENZYME-SYSTEM

Mammalian phenylalanine hydroxylase (PAH) catalyses the conversion of L-phenylalanine to L-tyrosine in the presence of dioxygen and tetrahyd robiopterin; it is a highly regulated enzyme. Little is known about th e rates of synthesis and degradation of PAH in vivo. The enzyme has be en reported to have a half-life of approx. 2 days in rat liver and 7-8 h in rat hepatoma cells, but the mechanism of its degradation is not known. In the present study it is shown that the tetrameric form of th e recombinant wild-type human enzyme is a substrate for the ubiquitin- conjugating enzyme system in the cytosolic fraction of rat testis. Our findings support the conclusion that multi-/poly-ubiquitination of hu man PAH plays a key role in the turnover of this cytosolic liver enzym e and provides a mechanism for the increased turnover observed for a n umber of recombinant mutant forms of the enzyme related to the metabol ic disorder phenylketonuria, when expressed in eukaryotic cells.