Abstract
Neurosporaxanthin (NX) is a carboxylic carotenoid produced by some filamentous fungi, including species of the genera Neurospora and Fusarium. NX biosynthetic genes and their regulation have been thoroughly investigated in Fusarium fujikuroi, an industrial fungus used for gibberellin production. In this species, carotenoid-overproducing mutants, affected in the regulatory gene carS, exhibit an upregulated expression of the NX pathway. Based on former data on a stimulatory effect of nitrogen starvation on carotenoid biosynthesis, we developed culture conditions with carS mutants allowing the production of deep-pigmented mycelia. With this method, we obtained samples with ca. 8 mg NX/g dry mass, in turn the highest concentration for this carotenoid described so far. NX-rich extracts obtained from these samples were used in parallel with carS-complemented NX-poor extracts obtained under the same conditions, to check the antioxidant properties of this carotenoid in in vitro assays. NX-rich extracts exhibited higher antioxidant capacity than NX-poor extracts, either when considering their quenching activity against [O2(1Δg)] in organic solvent (singlet oxygen absorption capacity (SOAC) assays) or their scavenging activity against different free radicals in aqueous solution and in liposomes. These results make NX a promising carotenoid as a possible feed or food additive, and encourage further studies on its chemical properties.
Highlights
Carotenoids are universally produced by photosynthetic species, but they are synthesized by heterotrophic organisms, such as many bacteria and fungi [1]
It was found that carotenoid production of carS mutants was about 4–5 times higher in a low-N than in a high-N medium [24], indicating that nitrogen is a negative regulator of the pathway
The results showed an increase in carotenoid content during the first three weeks of culture (Figure 1a), reaching approximately 6 mg of carotenoids/g dry mass
Summary
Carotenoids are universally produced by photosynthetic species, but they are synthesized by heterotrophic organisms, such as many bacteria and fungi [1]. In most of the cases investigated, it consists of the synthesis of β-carotene or some xanthophylls, such as astaxanthin, torularhodin and neurosporaxanthin [2] Some fungi, such as Blakeslea trispora and Xanthophyllomyces dendrorhous (formerly Phaffia rhodozyma) are biotechnological sources for the production of β-carotene and astaxanthin, respectively [3], but experimental facilities have made other fungi more suitable organisms to investigate carotenoid metabolism. Neurospora crassa and Fusarium fujikuroi, produce neurosporaxanthin (NX), a peculiar carboxylated carotenoid. This xanthophyll was described in N. crassa as an acidic pigment, found in a mixture with other carotenoids, and was later identified as the 35-C carboxylic apocarotenoid β-apo-40 -carotenoic acid [4]. The genetic analyses of albino N. crassa mutants led to the identification of the first genes involved in this biosynthetic pathway, opening the investigation of their orthologs in other fungi [2,6]
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