TYROSINE AND CYSTEINE ARE SUBSTRATES FOR BLACKSPOT SYNTHESIS IN POTATO

Partially purified blackspot pigments from potato tubers (Solanum tube rosum L.) of two commercial cultivars were subjected to a microassay f or melanin, which consisted of specific chemical degradation and subse quent HPLC analysis. Permanganate oxidation yielded pyrrole-2,3,5-tric arboxylic acid, whereas hydrolysis in hydriodic acid liberated aminohy droxyphenylalanine isomers. These results indicate that the polymeric pigments, which have previously been found to contain a protein matrix , carry crosslinked 5,6-dihydroxyindole-2-carboxylic acid and benzothi azine units. This leads to the conclusion that free tyrosine and free cysteine are incorporated in the proteinaceous pigments via the polyph enol oxidase catalysed pathway of melanogenesis in the process of blac kspot formation. The findings are in accordance with the hypothesis th at the process of blackspot formation is a non-regulated cascade of re actions in disintegrated tuber cells, rather than a finely tuned biosy nthesis. (C) 1998 Elsevier Science Ltd. All rights reserved.