| Tyrosine hydroxylase (TH) catalyzes the first and, more importantly, the rate-limiting step in the biosynthetic pathway of the catecholamine neurotransmitters dopamine, norepinephrine and epinephrine. The regulation of the biosynthesis of these neurotransmitters is vital to normal physiological and psychological function. The activity of TH is currently known to be regulated by three mechanisms: two short-term regulatory mechanisms (seconds to minutes), including end-product feedback inhibition and phosphorylation, and one long-term regulatory mechanism (12 to 48 hours), involving transcriptional level expression of additional TH enzyme which becomes available for utilization in the biosynthetic pathway. A proposed fourth type of regulation of TH is derived from the existence of two distinct forms of TH within neurons: cytosolic and neuronal membrane-bound. The proximity of the neuronal membrane-bound form of TH to the two sources of strong electric fields (100,000 V/cm) associated with cell membranes, transmembrane and surface potentials, was hypothesized to change the conformation of TH due to its internal structural electric field susceptibility, i.e., its polarizability. This study has shown that, in fact, TH activity is increased when exposed to an external electric field. Kinetic studies revealed Km values for both the L-tyrosine substrate and the 2-amino-4-hydroxy-6-methyltetrahydro-pteridine (6-MPH4) synthetic cofactor increased when TH was exposed to an external electric field. However, even though TH appears less efficient in the presence of an electric field, Vmax for both L-tyrosine and 6-MPH4 increase. The turnover number, kcat, of TH for L-tyrosine increased by 27% while the kcat of TH for 6-MPH4 increased by 41% in the presence of an applied external field. |
