Rat tyrosine hydroxylase has been expressed at high levels in Spodoptera frugiperda cells using a baculovirus expression system. A cDNA containing the coding region for PC12 tyrosine hydroxylase was inserted into the unique EcoRI site of the transfer vector pLJC8 to yield the recombinant vector pLJC9. Spodoptera frugiperda cells were then co-infected with pLJC9 and wild type Autographa californica nuclear polyhedrosis virus. Recombinant virus particles containing the cDNA for tyrosine hydroxylase were selected by hybridization with authentic tyrosine hydroxylase cDNA. Three recombinant viruses were plaque-purified. All expressed a protein of M(r) = 55,000 which reacted with antibodies to tyrosine hydroxylase. Forty-eight h after infection of cells with recombinant virus, the specific activity of tyrosine hydroxylase in the cell lysate was 30-100 nmol of dihydroxyphenylalanine produced/min/mg, consistent with 5-10% of the cell protein being tyrosine hydroxylase. Purification from 2.1 g of cells gave 5.8 mg of enzyme with a specific activity of 1.7 μmol of dihydroxyphenylalanine/min/mg. The purified enmzyme is a tetramer of identical subunits, containing one covalently bound phosphoryl residue and 0.1 iron atom/subunit. No carbohydrate was detectable. Steady state kinetic results with tetrahydrobiopterin as substrate are consistent with a sequential mechanism for binding of tyrosine and tetrahydrobiopterin. Substrate inhibition occurs at tyrosine concentrations above 50 μM. Steady state kinetic parameters at pH 6.5 are V(max) = 74 min-1, K(BH4) = 21 μM, K(Tyr) = 9.4 μM, and K(O2) ≤ 6 μM. The V(max) value shows a broad pH optimum around pH 7. The K(BH4) value is pH-dependent, increasing from about 20 μM below pH 7 to about 100 μM above pH 8. The K(Tyr) value is independent of pH between pH 6 and pH 8.5.
|Original language||English (US)|
|Number of pages||6|
|Journal||Journal of Biological Chemistry|
|State||Published - Aug 29 1990|
ASJC Scopus subject areas
- Molecular Biology
- Cell Biology