One promising system for the study of melanin structure is
provided by the human pathogenic fungus Cryptococcus neoformans.
Unlike most other melanotic fungi C. neoformans is unable to
synthesize melanin from endogenous precursors and it produces
only DOPA-melanin when grown on L-DOPA enriched media.
Consequently, cryptococcal cells are non-melanized unless provided
with the substrate, L-DOPA, for its specific melanin
synthetic enzyme laccase. C. neoformans has two major advantages
for the study of fungal melanin structure. First, the melanin
produced is all derived from a chemically defined exogenous
substrate added to the fungal culture thus providing the
opportunity for metabolic labeling to gain insight into the structure
and biochemistry of the pigment. For example, metabolic labeling
with 13C-DOPA has been used in combination with solid state
NMR to show that C. neoformans DOPA-melanin is covalently
linked to aliphatic compounds [8,9]. Secondly, acid digestion of
melanized cells yields hollow melanin spheres known as ‘ghosts’
that can be used to study the architecture of cell wall melanin [10].