The effect of tetrahydrobiopterin on the in situ phosphorylation of tyrosine hydroxylase in rat striatal synaptosomes.

Tetrahydrobiopterin (BH4), the obligatory cofactor of the aromatic amino acid hydroxylases, decreased the in situ 32P-phosphorylation of tyrosine hydroxylase (TH) in rat striatal synaptosomes. Incubation of pre-32P-labeled synaptosomes with BH4 in the presence of a permeant analogue of cAMP decreased the cAMP-stimulated level of 32P label incorporation into TH by about 50%, as determined by immunoprecipitation and autoradiography of SDS-polyacrylamide gels. The extent of inhibition mirrored changes in intrasynaptosomal BH4 levels and varied both as a function of BH4 concentration and length of incubation. A similar decrease in the amount of TH 32P-labeling was observed with the precursor of BH4, sepiapterin. This effect, in turn, was reversed by the inhibitor of sepiapterin reductase, N-acetyl-serotonin. Finally, exposure of pre-32P-labeled synaptosomes to the inhibitor of protein phosphatase 2A, okadaic acid, blocked the response to BH4. Collectively, the data suggest that BH4 stimulates the dephosphorylation of TH in situ and thus may play a dual role both as a cofactor for catalysis and a regulator of hydroxylase activity.