This review compiles the combined chemical and enzymatic synthesis of aromatic l-amino acids (l-phenylalanine, l-tyrosine, l-DOPA, l-tryptophan, and their derivatives and precursors) specifically labeled with carbon and hydrogen isotopes, which were elaborated in our research group by the past 20 years. These compounds could be then employed to characterize the mechanisms of enzymatic reactions via kinetic and solvent isotope effects methods.

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The synthesis of four isotopomers of dopamine labeled with deuterium or tritium is reported. The ring labeled [2 0 ,5 0 ,6 0 -2 H 3 ]-, and [2 0 ,5 0 ,6 0 -3 H 3 ]-dopamine were obtained using acid catalyzed isotopic exchange between dopamine and heavy or tritiated water respectively. Two selectively labeled isotopomers, i.e. [1R-2 H]-, and [1R-3 H]dopamine were synthesized by enzymatic decarboxylation of l-DOPA using the enzyme tyrosine decarboxylase (EC 4.1.1.25) from Streptococcus faecalis.

The kinetic (KIE) and solvent (SIE) isotope effect methods were used to investigate the mechanism of enzymatic hydroxylation of halogenated derivatives of l-tyrosine to l-DOPA catalyzed by the enzyme tyrosinase (EC 1.14.18.1). The values of deuterium KIE and SIE were obtained using the non-competitive method with spectrophotometric measurements. The Lineweaver–Burk plots were used for determination of the inhibition mode of 3′-iodo-l-tyrosine. Based upon kinetic effects values the mechanism of action of enzyme tyrosinase was proposed.

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