RESUMEN
Tyrosine hydroxylase (TH) is the key enzyme that controls the rate of synthesis of the catecholamines. SH-SY5Y cells with stable transfections of either human tyrosine hydroxylase isoform 1 (hTH1) or human tyrosine hydroxylase isoform 4 (hTH4) were used to determined the subcellular distribution of TH protein and phosphorylated TH, under basal conditions and after muscarine stimulation. Muscarine was previously shown to increase the phosphorylation of only serine 19 and serine 40 in hTH1 cells. Under basal conditions, the hTH1 and hTH4 proteins, their serine 19 phosphorylated forms and hTH1 phosphorylated at serine 40 were all similarly distributed; with ~80% in the cytosolic fraction, ~20% in the membrane fraction, and less than 1%, or not detectable, in the nuclear fraction. However, hTH4 phosphorylated at serine 71 had a significantly different distribution with ~65% cytosolic and ~35% membrane associated. Muscarine stimulation led to hTH1 being redistributed from the cytosol and nuclear fractions to the membrane fraction and hTH4 being redistributed from the cytosol to the nuclear fraction. These muscarine stimulated redistributions were not due to TH phosphorylation at serine 19, serine 40, or serine 71 and were most likely due to TH binding to proteins whose phosphorylation was increased by muscarine. This is the first study to show a difference in subcellular distribution between two human TH isoforms under basal and stimulated conditions.