The studies using
14
C and
3
H radioisotopes and
13
C and
2
H stable isotopes showed that 6MSA is
then modified extensively to form patulin. The products generated from m-cresol and gentisylaldehyde
are structurally similar to 6-methylsalicylic acid [19–21]. 6MSA is modified to m-cresol by 6MSA
decarboxylase, then the methyl group of m-cresol is oxidized to form an aldehyde group. This step is
followed by a hydroxylation reaction that leads to gentisaldehyde formation. However, at this point, it
seems that for over a decade the sequence of the different intermediates was much less clear. The
conversion of gentisaldehyde to a two ring structure such as patulin needs the opening of a ring by
a mechanism mediated either by a monooxygenase or by a dioxygenase. The isolation of several
patulin-minus mutants of P. griseofulvumled to the identification of four post aromatic precursors:
isoepoxydon [22] phyllostine [23], neopatulin [24] and ascladiol [25]. The discoveries of these
compounds led also to the hypothesis that an epoxidation step occurs after gentisaldehyde synthesis.
The nature of the epoxidation has remained a matter of speculation since a point of contention existed
as to whether the substrate of the epoxidation reaction was gentisyl alcohol or gentisaldehyde [26].
Indeed, although a kinetic pulse labeling study performed by Forrester and Gaucher [19] showed that
only the following co-metabolites are readily converted into patulin: acetate, 6MSA, m-cresol,
m-hydroxybenzyl alcohol, m-hydroxybenzaldehyde and gentisaldehyde, a crude extract which
catalyzed the epoxidation of gentisyl alcohol to phyllostine was isolated
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