Inhibition of Tyrosine Hydroxylase by R and S Enantiomers of Salsolinol, 1-Methyl-6,7-Dihydroxy-1,2,3,4- Tetrahydroisoquinoline

Salsolinol is one of the dopamine-derived tetrahydroisoquinolines and is synthesized from pyruvate or acetaldehyde and dopamine. As it cannot cross the blood-brain barrier, salsolinol as the R enantiomer in the brain is considered to be synthesized in situ in dopaminergic neurons. Effects of R and S enantiomers of salsolinol on kinetic properties of tyrosine hydroxylase [tyrosine, tetrahydrobiopterin:oxygen oxidoreductase (3-hydroxylating); EC 1.14.16.2], the rate-limiting enzyme of catecholamine biosynthesis, were examined. The naturally occurring cofactor of tyrosine hydroxylase, L-erythro-5,6,7,8-tetrahydrobiopterin, was found to induce allostery to the enzyme polymers and to change the affinity to the biopterin itself. Using L-erythro-5,6,7,8-tetrahydrobiopterin, tyrosine hydroxylase recognized the stereochemical structures of the salsolinols differently. The asymmetric center of salsolinol at C-1 played an important role in changing the affinity to L-tyrosine. The allostery of tyrosine hydroxylase toward biopterin cofactors disappeared, and at low concentrations of biopterin such as in brain tissue, the affinity to the cofactor changed markedly. A new type of inhibition of tyrosine hydroxylase, by depleting the allosteric effect of the endogenous biopterin, was found. It is suggested that under physiological conditions, such a conformational change may alter the regulation of DOPA biosynthesis in the brain.