Oxidative ring-coupling of tyrosine and its derivatives by purified rat intestinal peroxidase.

Intestinal peroxidase was shown to catalyse the oxidative ring-coupling of tyrosine, alpha-methyltyrosine, tyramine and morphine whereas amphetamine was not oxidized to any detectable extent. The oxidative ring-coupling reaction can be monitored by changes in absorbance spectra and the dimers formed in this way with morphine and alpha-methyltyrosine were identified by mass spectrometry. Intestinal peroxidase also catalysed the peroxidatic oxidation of L-DOPA and alpha-methyl-L-DOPA, but in this case the reaction would be expected to be more complicated and to yield a variety of possible products. The kinetic parameters for the oxidation of each of these substrates were determined. Since the products of the oxidative ring-coupling reactions may have different pharmacological properties to those of the parent compounds, these studies suggest that, in the presence of an adequate supply of metabolically produced hydrogen peroxide, the action of intestinal peroxidase may affect the behaviour and pharmacokinetics of these compounds after oral administration.