The enniatin complex
The enniatins are a group of currently 28 known cyclic depsipeptides that are naturally produced by filamentous fungi in the genera Fusarium, Halosarpheia and Verticillium (Figure 1). All naturally occurring enniatins consist of three D-hydroxy acid residues and three L-amino acid residues linked alternatively with ester and amide bonds forming an 18-membered cyclic molecule. The amino acids that mostly have been found to be incorporated into the enniatin molecule include N-methyl-valine, N-methyl-isoleucine and N-methyl-leucine. Additionally, it has been reported that N-methyl-alanine, N-methyl-threonine and N-methyl aminobutyric acid can be incorporated into enniatins by Fusarium species.
Enniatins possess a wide range of biological activities. These substances are known as ionophores, phytotoxins, anthelmintic compounds and antibiotics. The antibiotic effects of enniatins have been used in a pharmaceutical commodity with anti-inflammatory properties, called fusafungine. Fusafungine has found applications in treatment of respiratory tract infections and reportedly has a positive effect on wound healing after tonsillectomy.
Literature data repeatedly report the substantial cytotoxicity associated with exposure of enniatins to different types of cells. The inhibitory concentrations (IC50) in general cytotoxicity assays are commonly observed in the lower µM-range. Recently, several studies have indicated that enniatins exert their cytotoxic activities, in addition to being ionophores, through the induction of mitochondrial modifications and cell cycle disruption, finally resulting in apoptotic cell death. Enniatins are also specific inhibitors of acyl-coenzyme A cholesterol acyltransferase (ACAT) activity. Recent research demonstrated the potential use of enniatins as anticancer drugs since they appear to be more toxic in cancer cell lines. Of further interest for pharmaceutical applications may be the reported interaction of enniatins with the multidrug resistance protein Pdr5p in Saccharomyces cerevisiae at nontoxic concentrations. A similar behavior has been shown towards human Pgp, MRP2 and BCRP in Caco-2 cells.
However, enniatins are also widespread contaminants in grains and cereals due to infection with enniatin-producing Fusarium species. Cereals in Northern Europe were contaminated with high levels of enniatins in recent years adding up to maximum concentrations of 7.7 mg/kg and 24.8 mg/kg in Norwegian and Finnish wheat, respectively. In Mediterranean countries, wheat and sorghum containing up to 493 mg/kg and 696 mg/kg enniatins, respectively, were observed. Even though enniatins apparently are of low acute toxicity in vivo, their effects after long-term exposure and in combination with other mycotoxins remain unknown. The lipophilic nature of the enniatins might also lead to their bioaccumulation in animal and human tissues. The lack of correlation between toxicity data in vitro and in vivo is likely explained by their low bioavailability upon oral uptake and fast metabolism and excretion, which has recently been shown in studies conducted at the Norwegian Veterinary Institute.
Figure 1. Overview over the presently known natural enniatin analogues. Still more analogues are known from precursor-directed biosynthesis