The metabolism of beta-(3,4 dihydroxyphenyl)-L-alanine by plants

Abstract

Methods for the detection and characterisation of DOPA derivatives in plant tssues have been investigated. In relation to this study, several methods for the production of possible naturally occurring derivatives were explored. The most success was achieved by feeding L-DOPA to Pislum sativum seeds which were then allowed to germinate for a short periods. Five compounds were isolated from the treated seedlings and one of them was shown to be identical to the DOPA glucoside which had previously been reported in the testas of broad been (Vicia faba) seeds (5). The same compound was later identified in the cotyledons of dormant bean seeds, and its structure shown to be B - (3-B-D - glucopyranosyloxy) - 4 - hydroxyphenyl) - L - alanine. The chemical nature of the other compounds was also investigated and tentative deductions made for their structures. A number of 0 and N - methylated derivatives of DOPA have been synthesised chemically. Their properties have been recorded and whereby isomeric 0 - substituted DOPA derivatives may be distinguished are outlined.Using the information from the preliminary studies, the distribution of DOPA and its derivatives in about 200 different species of plants have been investigated. This has revealed that DOPA derivatives are rare in plants, although free DOPA has been detected in a number of leguminous species and in species grouped within the order of Centrospermae. This latter discovery adds weight to the suggestion that DOPA is a precursor of the characteristic red-violet and yellow pigments which are found within the Centroapermae.A detailed study the distribution of DOPA and related compounds in the broad bean has been undertaken. Variations in the concentrations of the compounds throughout the growth cycle have been studied qualitatively using chromatographic and electrophoretic techniques, and quantatively, using an iodine colourimetric procedure and by ran exchange chromatography. The presence of DOPA in the tissues of leguminous species is often associated with the formation of dark brown or black pigments. Observations on the biogenesis, histology and chemical nature of these pigments are described. Comparison of the chemical and physical properties of plant pigments with those of biosynthetically prepared melanins showed that there are considerable differences. The plant pigments do not appear to be similar to animal melanins. The enzyme in dandelion (Taraxacum officinale) leaves which catalyses the deamination of DOPA has been studied. This appears to be labile, particulate and distinct from the phenylalanine ammonia lyase which is also present in the leaves.