The relative roles of phenylalanine and tyrosine as substrates for DOPA synthesis in PC12 cells.

The relative contributions of tyrosine (TYR) and phenylalanine (PHE) to the synthesis of dihydroxyphenylalanine (DOPA) were studied in PC12 cells following inhibition of aromatic L-amino acid decarboxylase with m-hydroxybenzylhydrazine (NSD-1015). Cells were incubated with varying concentrations of unlabeled L-TYR and L-PHE, and either L-(3)H-TYR or L-(3)H-PHE. Following incubation, labeled and unlabeled TYR, PHE, and DOPA were quantitated following HPLC separation. PC12 cells synthesized DOPA from both TYR and PHE. Raising the concentration of one amino acid relative to that of the other increased the proportion of DOPA synthesized from that amino acid. TYR suppressed DOPA synthesis from (3)H-PHE at concentrations lower than that observed for a similar inhibition by PHE of DOPA synthesis from (3)H-TYR. Inhibition of total DOPA synthesis occurred only at high concentrations of either amino acid. The results suggest that in the PC12 cell, TYR and PHE can be used interchangeably as substrates for TYR hydroxylation, and that the proportion of catecholamine synthesized will depend on the relative proportions of each substrate available to the cell. However, TYR is clearly the preferred substrate for tyrosine hydroxylase.