Patulin and Penicillic Acid

Two interesting metabolites of Penicillium patulum are patulin and penicillic acid. Their biosynthesis involves the cleavage of an aromatic ring. These substances are mycotoxins and their activity in this context is discussed in Chapter 9. 

2-carboxylic add (4.11). This decomposition suggested the presence of a masked p-formylketone in the structure. Earlier formulations for patulin were eventually corrected by Woodward in 1949, who synthesized deoxypatulin and, in 1950, patulin itself, using tetrahydro-4-pyrone as a starting material. 

It is interesting to speculate how quickly the structure might have been determined had NMR spectroscopy been in widespread use at the time. Indeed infrared correlations, which were being developed at the time of Woodward s proposal for the structure of patulin, were in accord with the structure. The presence of the enol of a ketone and a ketal in the structure also play an important part in the biological activity of patulin. 

The biosynthesis of patulin was also studied using 02]acetate and 2 gas and then measuring the oxygen-18 induced chemical shifts in the NMR spectrum to locate the site of the This showed that only the carbonyl oxygen was derived from acetate and that the others were derived by oxidative processes. Many of the enzyme systems that mediate these steps have been isolated from P. patulum. 

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Zamir, L. O., The biosynthesis of patulin and penicillic acid, in The Biosynthesis of Mycotoxins (P. Steyn, ed.), 223-268, Academic Press, New York, 1980. 

Patulin and penicillic acid are unsaturated lactones and are inactivated by cysteine. They are also unsaturated ketones whether they react with cysteine as lactones or as ketones is not known. It has been suggested (48,78) that the unsaturated lactones and ketones that have antibacterial activity react with sulfhydryl and possibly with amino groups of essential enzyme proteins. All of the substances listed above under ketone and lactone, with the possible exception of helvolic acid for which there is no data, are inactivated by cysteine. 

Patulin (1) and penicillic acid (2) are secondary metabolites which are synthesized primarily by Penicillium and Aspergillus species. The patulin isolated has been given various names patulin, claviformin, clavacin, clavatin, expansin, leucopin, mycoin C, penicidin, and tercinin (Florey et aL, 1949 Scott, 1974 Singh, 1967 Wilson and Hayes, 1973 Wilson, 1976). Patulin and penicillic acid are produced by different fungal species, but the biological reactions leading to these two natural products are very similar. In this chapter, only the physical properties of patulin and penicillic acid relevant to biosynthetic studies will be discussed. 

Whole-cell feeding experiments imply adding labeled putative precursors (radioactive or stable isotope label) to the whole organism, which is grown on a synthetic media. The products and intermediates are then isolated, purified, and analyzed by the various methods. This type of experiment enables one to propose a plausible biosynthetic pathway. Isolation and purification of the enzymes responsible for each step of the pathway enable verification at the enzymatic level. The in vitro conversion of a putative precursor by the pure enzyme to the product constitutes unambiguous proof of the biosynthetic reaction. The ultimate proof of a biosynthetic pathway depends, therefore, on the characterization of the enzymes involved in each step of the pathway. The only major drawback is that the procedure is extremely difficult, and the detailed enzymology for polyketide-derived eompounds deserves further study. The problems are (a) instability of the enzymes, (b) lack of reproducibility, and (c) variability of activity in different enzymatic preparations (some of these enzymes are membrane bound and easily deactivated by isolation). Despite the difficulties involved, some of the enzymes involved in the biosynthesis of patulin and penicillic acid have been partially purified and characterized. 

The feeding of a putative precursor can sometimes result in the production of an unusual metabolite which otherwise is not a natural product. This metabolite does not represent the in vivo processes and is only a derailment product or forced metabolite. In the case of patulin and penicillic acid, these derailment products are 6-ethylsalicylic acid, homoorsellinic acid, and deoxyepoxidon. 

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