, Volume 208, Issue 1, pp 114-124

Tyrosinase involved in betalain biosynthesis of higher plants

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A tyrosine-hydroxylating enzyme was partially purified from betacyanin-producing callus cultures of Portulaca grandiflora Hook. by using hydroxyapatite chromatography and gel filtration. It was characterized as a tyrosinase (EC and EC by inhibition experiments with copper-chelating agents and detection of concomitant o-diphenol oxidase activity. The tyrosinase catalysed both the formation of L-(3,4-dihydroxyphenyl)-alanine (Dopa) and cyclo-Dopa which are the pivotal precursors in betalain biosynthesis. The hydroxylating activity with a pH optimum of 5.7 was specific for L-tyrosine and exhibited reaction velocities with L-tyrosine and D-tyrosine in a ratio of 1:0.2. Other monophenolic substrates tested were not accepted. The enzyme appeared to be a monomer with an apparent molecular mass of ca. 53 kDa as estimated by gel filtration and SDS-PAGE. Some other betalain-producing plants and cell cultures were screened for tyrosinase activity; however, activities could only be detected in red callus cultures and plants of P. grandiflora as well as in plants, hairy roots and cell cultures of Beta vulgaris L. subsp. vulgaris (Garden Beet Group), showing a clear correlation between enzyme activity and betacyanin content in young B. vulgaris plants. We propose that this tyrosinase is specifically involved in the betalain biosynthesis of higher plants.

Received: 14 July 1998 / Accepted: 23 October 1998