Cynodontin, emodin, endocrocin and secalonic acids A, E and G have been isolated from five strains of Pyrenochaeta terrestris. Aspergillus aculeatus produces emodin, endocrocin and secalonic acids B, D and F. No cynodontin was detected. Isolation of emodin in small amounts supports previous evidence that it is an intermediate in secalonic acid biosynthesis. Isolation of cynodontin and endocrocin, which are co-produced with secalonic acids in other organisms, suggests that these compounds are formed by a common branching pathway. A natural isolate of P. terrestris contained variant strains which produced different relative amounts of secalonic acids A, E and G. From the combinations of secalonic acids produced in organisms so far examined it is concluded that precursor tetrahydroxanthone units are formed in pairs differing in stereo-chemistry only at C-5 or at the traps-invariant C-6 : C-10a positions. A possible biosynthetic pathway is discussed. Sccalonic acids are members of the ergochrome group of secondary metabolites, reviewed by FRANCK and FLASCH". Seven different kinds have been isolated from fungi and lichens (Fig. I). Secalonic acids A (I), B (2) and C (3) were obtained from Claviceps purpurea ", secalonic acids A. E (4) and G (5) from Pj'renoc/raeta terrestris',", and secalonic acids D (6) and F (7) from Aspergillus aculeaiuss'. Secalonic acids A and C have also been obtained from Cetraria ornata ', secalonic acid A was found in Asnerg/l/us ochraceu.s3> and Parmelia eutotheiochroa°', and secalonic acid D was isolated from Pemci//ium o.valicum'". We report here, in addition, the isolation of secalonic acid B from A. aculeatus strain MIT-M29. In C. purpurea and P. oxalicum secalonic acids are reported to incorporate label from ["C]emodin and [3H]emodin but not from ["C]endocrocin'•'". Lack of incorporation from the latter is not unexpected since STEGLtCH et al.1 have shown that, in Dermocrbe .ranqainea, emodin is formed independently and not by decarboxylation of endocrocin. That emodin is a precursor is more surprising, since this compound possesses a C-3 hydroxyl that is not present in the secalonic acids. Both emodin (8) and endocrocin (9) have been found in Dy sanguinea and D. semisaiiguinea''', Penicilliutu i.slcanlicum"' and Penicf/lium tardum'". Endocrocin has been isolated along with secalonic acids from C. purpurea''° and Cetraria ora(tta'). However, endocrocin has not been identified in other secalonic acid-producers and there is no evidence that emodin is a normal metabolite of C. purpurea and P. o.valicutn. We NRCC No. 17754.
The pattern of 13C and 2H incorporation from [1-13C]-, [2-13C]-, [1-13C0;1;1,2-13C1;0;1]-, and [2-13C0;1,2-2H3;3]acetate into dihydrofusarubin 1, produced by cultures of Fusariumsolani, has been determined by 13C and 2H nmr of the derivatives anhydrofusarubin 3 and anhydrofusarubin diacetate 4. The results show that 1 is biosynthesized from seven uniformly-incorporated acetate units with C-3, C-11 originating from the "starter" unit. They strongly suggest that linear head-to-tail condensation of an acetate and six malonate units forms a single-chain heptaketide intermediate. The evidence also suggests that, during conversion of [13C, 2H]-labeled acetyl-CoA to malonyl-CoA, 2H is transferred to biotin carboxyl carrier protein where it does not exchange rapidly with the medium and is available for conversion of endogenous malonyl-CoA to [2H]-enriched acetyl-CoA.
Elsinochromes C and D have been isotopically labelled by supplementing cultures of the producing fungus with sodium [1-13C]-, [2-13C]-, [1,2-13C]-, or [2-13C, 2-2H3]acetate and the distribution of isotope has been determined by 13C and 2H nuclear magnetic resonance spectroscopy. The pattern of 13C labelling is consistent with assembly of the elsinochrome carbon skeleton from two heptaketide chains with loss of the terminal carboxyl groups, and dimerization to generate the 1,2-dihydrobenzo (ghi)-perylene ring system. Elsinochrome D was converted to the more soluble triacetate to examine the fate of 2H presented to the culture as [2-13C, 2-2H3]acetate. Deuterium labelling was restricted to the C-14 and C-16 methyl groups, a result consistent with these belonging to the "starter" C2 units of polyketide intermediates. Carbon-13 enrichment data for elsinochromes labelled from [1,2-13C]acetate indicated that the polyketide chains were formed from a highly enriched precursor pool with subsequent dilution accounting for a much lower average incorporation. Presence of spin–spin coupling between linked carbons derived from separate polyketide precursors implies that dilution occurs after the dimerization step. From the combined evidence a plausible pathway for elsinochrome biosynthesis is deduced.
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