Mannitol is not involved in protective reactions of Acanthamoeba

Genes for mannitol-metabolizing enzymes, mannitol phosphate dehydrogenase (MPDH) and mannitol dehydrogenase (MDH), have been recently identified in the genome of Acanthamoeba castellanii and their potential role in stress tolerance was proposed. Using qRT-PCR, comparison has been made of mRNA levels of the enzymes for mannitol metabolism at various time intervals during the stress defence reactions of encystation and pseudocyst formation. Gradual decrease of both enzymes during encystation and slight increases at the beginning of pseudocyst formation were observed. Detailed analysis of mRNA sequences of the two genes revealed similarities with various alcohol dehydrogenases rather than mannitol dehydrogenases. Our results indicate there is probably no protective role for mannitol in Acanthamoeba as no mannitol was detected using HILIC ESI MS, in any Acanthamoeba life cycle stage. Possible misinterpretation of previously published sequences as encoding enzymes of the mannitol metabolic pathway is discussed.