The major yellow water soluble pigment in basidiocarps of many Hygrocybe spp. is muscaflavin (Steglich and Strack 1990), an unusual betalain pigment first identified as a minor pigment in A. muscaria (Steglich and Preuss 1975; Von Ardenne et al. 1974). Cibula (1976) partially characterized P505-15 the same pigment calling it flavohygrocybin. Muscaflavin comprises a 7-membered heterocyclic ring, formed by the action of a 2,3- DOPA dioxygenase on DOPA followed by spontaneous recyclization of the resulting 2,3-seco-DOPA
intermediate (Steglich and Preuss 1975; Von Ardenne et al. 1974) (Fig. 4). Betalamic acid is also present in A. muscaria and H. conica (Musso 1979; Terradas and Wyler 1991a, b). Examination of the peptide sequences of the fungal, bacterial and plant DOPA dioxygenases shows little similarity, suggesting that these pathways have all evolved independently (Grotewold 2006; Novotna et al. 2004). Whilst the major red pigments of Amanita muscaria (e.g. muscapurpurin) are derived from betalamic acid, the orange-red
pigments of Hygrocybe spp. (hygroaurins) are apparently derived from muscaflavin via conjugation with amino acids. Bresinsky and Kronawitter (1986) confirmed the involvement of threonine but the precise nature of the red pigment(s) remains unknown. Cibula (1976) partially characterized a magenta pigment (‘rhodohygrocybin’, find more a type of hygroaurin), which was quantitatively correlated with the redness of the pileus, and he also noted its chemical similarity to muscaflavin (with these two pigments accounting for >80 % of the light absorption of pilei). Thus with muscaflavin (flavohygrocybin sensu Cibula) absorbing
light below 500 nm (reflecting light at 500–700 nm –i.e., yellow) and ‘rhodohygrocybin’ absorbing light at 480–590 nm, the combined effect of these pigments is reflection of bright this website red. Cibula also found that muscaflavin was present at much higher concentrations (ca. 1200 ppm) than ‘rhodohygrocybin’ (ca 60 ppm) even in species with bright red pilei, with the latter also being less stable (Online Resource 4). The presence of an amino group (ninhydrin positive) in rhodohygrocybin further suggests that it is a hygroaurin, as discovered by Bresinsky and Kronawitter (1986), possibly conjugated with cyclo-DOPA (as found in betanidin) or an aromatic amino acid to achieve absorbance in the 500–600 nm region. The blackening of older or bruised basidiocarps of H. conica is also linked to muscaflavin synthesis, probably the result of melanin formation following oxidation of DOPA to DOPA-quinone and ultimately melanin by tyrosinase (Steglich and Preuss 1975).