Citrinin biosynthesis

The main concern regarding the utilization of Monascus pigments relates to the production of the citrinin mycotoxin in Monascus cultures. Several methods for controlling the mycotoxin production were proposed, including selection of non-toxinogenic strains, control of citrinin biosynthesis, and modifications of culture conditions. Despite their wide and traditional food applications in Asian countries, Monascus pigments have not been approved for use in the United States or European Union. 

Feeding experiments with isotopically labeled precursors have shown that many NR fungal polyketides are formed by the use of advanced starter units. In the classic case of norsolorinic acid 7 biosynthesis, it has long been known that hexanoate forms the starter unit. Differential specific incorporation of acetate into the early and late positions in compounds such as citrinin 3 have been used to argue that these compounds may have been formed by more than one PKS so that one PKS makes an advanced starter unit, which is passed to a second PKS for additional extension. 

Similar domains are known from NRPS systems in which reductase domains are sometimes used as chain release mechanisms, which release an aldehyde or primary alcohol. In the case of MOS and citrinin biosynthesis the reductive release mechanism makes good sense as this provides the products with C-1 at the correct oxidation state (Fig. 2c). 

Pisareva, E., Savov, V., and Kujumdzieva, A. 2005. Pigments and citrinin biosynthesis by fungi belonging to genus Monascus. Z. Naturforsch [C] 60(1-2), 116-120. 

Shimizu, T., Kinoshita, H., Ishihara, S., Sakai, K., Nagai, S., and Nihira, T. 2005. Polyketide synthase gene responsible for citrinin biosynthesis in Monascus purpureus. Appl. Environ. Microbiol. 71(7), 3453-3457. 

Citrinin (9.22) is a widespread mycotoxin that is formed by many Penicillium and Aspergillus species, particularly P. citrinum, P. expansum and P. roqueforti. It was first isolated in 1931 by the Raistrick group and described as an antibiotic. It has been shown to have carcinogenic effects and to produce kidney damage. Evidence for its structure and biosynthesis is discussed in Chapter 4. 

Biosynthesis of citrinin by Penicillium citrinum, J. Barber, R.H. Carter, M.J. 

The specificity of the inhibition by mevinolin through its high affinity for HlK-CoA reductase may also serve as a model to develop highly effective and specific artificial biocides and to stinulate research in this topic. For example, citrinin, another antibiotic having a bicyclic structure produced by Pentctllium citrinum has been shown to inhibit sterol biosynthesis (143) at the site of acetoacetyl-CoA thiolase (EC 2.3.2.9) and HMG-CoA... 

Curtis RF, Hassall CH, Nazar M (1968) Biosynthesis of Phenols. XV. Metabolites of Penicillium citrinum Related to Citrinin. J Chem Soc C 85... 

Hajjaj reported the biosynthetic pathway of citrinin in M. ruber. The tetraketide, obtained from the condensation of one acetyl-CoA molecule and three malonyl-CoA molecules, is the precursor for the citrinin biosynthesis (Figure 6.31). The tetraketide, along with an additional acetyl-CoA molecule, generates the intermediate 31, which through a series of reactions, including methylation, reduction, oxidation, and dehydration, forms citrinin [147],... 

Figure 6.31 Biosynthesis of citrinin from Monascus ruber.

The biosynthesis of ascochitine (16), a phytotoxic hexake-tide from the fungus Ascochyta fabae, has been investigated by means of C-labeled acetates and methionine and Relabeled precursors (Ballio, 1981 Stoessl, 1981) (Fig. 5.12). Ascochitine is derived from a single hexaketide precursor with introduction of three methyl groups from methionine via a quinone-methide structure (17) similar to that involved in the biosynthesis of citrinin. The aldehyde (18) and quinone-methide (17) were specifically incorporated. 

Barber, J., and J. Stauton Protium as a Tracer in Polyketide Biosynthesis Incorporation of CH3 C02H into Citrinin Produced on a Medium Based on D2O. Chem. Commun. 1979,1098. 

Carter, R.H., M.J. Garson, and J. Staunton Biosynthesis of Citrinin Incorporation Studies with Advanced Precursors. Chem. Commun. 1979, 1097. 

Colombo, L., C. Gennari, D. Potenza, C. Scolastico, F. Aragozzini, and C. Merendi Biosynthesis of Citrinin and Synthesis of its Biogenetic Precursors. J. Chem. Soc. Perkin Trans. 1 1981, 2594. 

Curtis, R.F., C.H. Hassal, and M. Nazar The Biosynthesis of Phenols Part XV. Some Metabolites of Penicillium citrinum related to Citrinin. J. Chem. Soc. (C) 1968, 85. 

The biosynthesis of citrinin (XXXIX) was independently investigated with radioactive tracers by Birch et al. (1958c) utilizing Aspergillus candidus] and by ScHWENK and collaborators (1958) with labelled acetate and methionine-methyl-in Penicillium citrinum cultures. It was unequivocally demonstrated that this compound arises by cyclization of a poly j8-ketide made of 5 acetyl units, and that its two extra methyls and the carboxyl function arise from the Ci-pool (Fig. 16). It has subsequently been suggested (Hassall and Jones, 1962) that (XL) and 3>4-dihydro-6,8-dihydroxy-3,4,5"trimethylisocoumarin-carboxylic acid (XLI) are precursors to citrinin. This postulation followed the finings cf. Hassall, 1965) that one mutant strain of A, terreus produced only (XLI),... 

Birch, A. J., P. Fitton, E. Pride, A. J. Ryan, H. Smith, and W. P. Whalley Studies in relation to biosynthesis. Part. XVII. sclerotiorin, citrinin and citromycetin. J. Chem. Soc. 1958c, 4576. 

---