Non-protein amino acids and neurodegeneration: the enemy within

Animals, in common with plants and microorganisms, synthesise proteins from a pool of 20 protein amino acids (plus selenocysteine and pyrolysine) (Hendrickson et al., 2004). This represents a small proportion (~2%) of the total number of amino acids known to exist in nature (Bell, 2003). Many 'non-protein' amino acids are synthesised by plants, and in some cases constitute part of their chemical armoury against pathogens, predators or other species competing for the same resources (Fowden et al., 1967). Microorganisms can also use selectively toxic amino acids to gain advantage over competing organisms (Nunn et al., 2010). Since non-protein amino acids (and imino acids) are present in legumes, fruits, seeds and nuts, they are ubiquitous in the diets of human populations around the world. Toxicity to humans is unlikely to have been the selective force for their evolution, but they have the clear potential to adversely affect human health. In this review we explore the links between exposure to non-protein amino acids and neurodegenerative disorders in humans. Environmental factors play a major role in these complex disorders which are predominantly sporadic (Coppede et al., 2006). The discovery of new genes associated with neurodegenerative diseases, many of which code for aggregation-prone proteins, continues at a spectacular pace but little progress is being made in identifying the environmental factors that impact on these disorders. We make the case that insidious entry of non-protein amino acids into the human food chain and their incorporation into protein might be contributing significantly to neurodegenerative damage.