The Effect of Ethylene Glycol Bis(β-aminoethyl ether)-N,N'-tetraacetic Acid and Calcium on Tyrosine Hydroxylase Activity

It has been suggested that calcium plays a role in the regulation of tyrosine hydroxylase [tyrosine 3-monooxygenase; L-tyrosine,tetrahydropteridine oxygen oxidoreductase (3-hydroxylating); EC 1.14.16.21 activity. We have studied the effects of calcium and the calcium-chelating agent ethylene glycol bis(β-aminoethyl ether)- N,N '-tetraacetic acid (EGTA) on tyrosine hydroxylase activity in vitro . The enzyme was assayed with both optimal and suboptimal concentrations of substrate and cofactor, and with both a chemical and an enzymatic system for maintaining the cofactor in the reduced state. We did not observe any significant stimulation by EGTA of tyrosine hydroxylase in crude extracts prepared from rat striatum, hypothalamus, or adrenal glands. In addition, there was no appreciable effect of calcium on tyrosine hydroxylase in crude striatal extracts. These results do not support the hypothesis that EGTA stimulates tyrosine hydroxylase in dopaminergic neurons.