Contact allergy to Australian blackwood (Acacia melanoxylon R.Br.): isolation and identification of new hydroxyflavan sensitizers.

Australian blackwood is known to be an important cause of allergic contact dermatitis in Australia. Previous investigations have shown that 2,6-dimethoxy-1,4-benzoquinone and acamelin are 2 of the responsible, but weak, sensitizers. When these 2 quinones are lacking, which is occasionally found, the wood still possesses allergenic properties. The present re-examination led to the isolation and identification of 4 hydroxyflavans, of which 3 proved to be allergens. Melacacidin, known to be the main constituent of these flavan derivatives in the heartwood, was isolated and its sensitizing capacity in guinea pigs determined. It showed a moderate sensitizing power. Melacacidin occurs abundantly in 125 Australian and 3 African Acacia species.