Transketolase from the leaves of Nicotiana tabacum var. John William′s Broadleaf is inhibited by exposure to low intensity blue light in the presence of low concentrations of FMN, which functions as a sensitizer. The degree of photoinactivation is reduced in the presence of quenchers of triplet FMN. Photoinactivation does not occur in the absence of oxygen and is also lessened in the presence of quenchers of singlet oxygen. The mechanism of photooxidation is inferred to proceed by way of

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... FMN and a major route of inactivation of the enzyme may continue via singlet oxygen.