Laetisaric acid

Laetisaric acid, a hydroxylated fatty acid isolated from a soil fungus, is described. Its structure-activity relationships led to the design of even more active and simpler compounds. This example provides another illustration of the value of natural compounds and the valuable information that they provide. 

Fig. 1 Radial growth of P. ultimum over time with and without 20 ng/ml laetisaric acid in the growth medium.

SILICA PREP TLC (BENZENE/ETHYL ACETATE/FORMIC ACID 80 20 1 ) ACTIVE LAETISARIC ACID (RF 0.4)... 

Fig. 3 Isolation of laetisaric acid from Laetisaria arvalis.

The amount of laetisaric acid in a sample can be readily quantified using the above extraction, thin layer separation, diazomethane derivatization and capillary gas chromatographic procedure. Calculations of the amount of laetisaric acid by bioassay and by the described physical analytical methods are in agreement. 

The 70 eV electron impact mass spectrum of laetisaric acid methyl ester gives a base peak at m/z 93 and a strong peak at m/z 292 due to a loss of water from the molecular ion at m/z 310. This indicates a molecular formula of C1gH3403. The 400 MHz NMR analysis of laetisaric acid methyl ester in deuterochloroform is characteristic of a fatty acid methyl ester of 34 protons with the presence of a nonconjugated dienol system 4.45 (1H,dt,J=8.4,6.3Hz), 5.31... 

Mass spectra of the trimethylsilyl derivative of laetisaric acid gives an ion peak at m/z 239 corresponding to C5H11CH=CHCH2-CH=CH-CH=OTMS and the mass spectra of the oxidized derivative gives a m/z at 165 for... 

C5HhCH=CHCH2CH=CHC=0. These fragments indicate a hydroxyl at the C-8 position in laetisaric acid. 

We synthesized laetisaric acid to confirm the assigned structure and to produce a sufficient quantity of active material for biological studies. The Grignard coupling of... 

To explain these data we developed the hypothesis that laetisaric acid and active analogs are metabolized by sensitive fungi, such as P. ultimum, via common fi-oxidation to an active 2-hydroxy twelve carbon fatty acid. In the case of laetisaric acid the metabolic product is 2-hydroxydodecadienoic acid. This a-hydroxy compound is apparently not further metabolized by fi-oxidation and accumulates as the ultimate allelopathic agent. 

We have speculated on but do not understand the mechanism causing the lytic activity of laetisaric acid. The active twelve carbon metabolite of laetisaric acid may poison a key enzyme in lipid metabolism or disrupt the integrity of the fungal cell membrane by insertion or dissolution as has been shown in Escherichia coli with sodium dodecyl sulfate and Triton X-100 (24 r 25). Why the C-12 molecule is most active remains to be determined. Kinetic studies of lipid metabolism and physicochemical and ultrastructural investigations of membranes treated with the putative active metabolite may answer these questions. 

The hydroxylated fatty acids hold the promise of being safe, simple and specific plant protectants. Our work on laetisaric acid demonstrates how the investigation of allelochemical interactions may lead to the development of new biorational agrochemicals. 

From a practical point of view, the antibiotic activity of fatty acids and their salts is interesting given that they are regarded as non-toxic to most vertebrates and higher plant species. The possibility of incorporating the genetic mechanism for the production of laetisaric acid into crop plants or soil bacteria would seem worthy of investigation. 

An illustration of the various activity that fungal metabolites can have is illustrated by laetisaric acid which has also been found to be a sporogenic factor in Aspergillus nidulans. Laetisaric acid (11) and 8-hydroxystearic acid together with the corresponding 1,5-lactones (18 and 19) induce premature sexual sporulation in A. nidulans [22]. 

The 0R-8E, 2Z- and 2R-9Z, 135-derivatives of linoleic acid (20, 21) and 9R-10E- and the 10/ -8 -hydroxyderivatives of stearic acid (22, 23) have been isolated as compounds with fungitoxic activity towards Cladosporium phlei [23]. As suggested above for laetisaric acid, the possibility that the corresponding hydroperoxides are antifungal agents should be considered. 

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