Quinonoid constituents as contact sensitizers in Australian blackwood (Acacia melanoxylon RBR)

Abstract

Australian blackwood (Acacia melanoxylon RBR) is a valuable commercial timber that since 1925 has been incriminated as being injurious to health. In addition to toxic effects numerous cases of allergic contact dermatitis and bronchial asthma have been observed in woodworkers. Several constituents have been identified in recent years, but none of them could be considered as aetiological factors. Sensitizing experiments performed with blackwood heartwood extracts corroborated the described sensitizing properties. Chemical studies showed the occurrence of two or possibly three quinones that produced positive skin responses in the sensitized guinea pigs. The main contact allergens were isolated and identified by X-ray analysis. The first, a yellow quinone, was identified as 2,6-dimethoxy-1,4-benzoquinone while the second, a red quinone, has the structure of 6-methoxy-2-methyl-3,5-dihydrobenzofurano-4,7-dion and was named acamelin. Whereas 2,6-dimethoxy-p-benzoquinone is already known from natural sources, acamelin is new and belongs to the rare group of naturally occurring furanoquinones. Though the obtained sensitizing capacity of A melanoxylon RBR in respect of its quinones is not high, it should be considered as a possible source of allergic contact dermatitis, especially as greater amounts of Australian blackwood may enter European countries in the near future.