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

摘要

Tetrahydrobiopterin (BH4), the obligatory cofactor of the aromatic amino acid hydroxylases, decreased the in situ32P-phosphorylation of tyrosine hydroxylase (TH) in rat striatal synaptosomes. Incubation of pre-32P-labeled synaptosomes with BH4in the presence of a permeant analogue of cAMP decreased the cAMP-stimulated level of32P 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 BH4levels and varied both as a function of BH4concentration and length of incubation. A similar decrease in the amount of TH32P-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 BH4stimulates 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.