Trace amines are a chemical group of amines and their isomers, included in the biogenic amines’ family. They are natural compounds with low molecular weight, formed during the natural metabolism of animals, plants, and microorganisms from amino acids precursors.
1. Introduction
Trace amines are a class of natural endogenous amines with low molecular weight, formed during the natural metabolism of animals, plants, and microorganisms from amino acids precursors
[1]. In general, the trace amine family includes all endogenous amines present at a low nanomolar concentrations in invertebrate and vertebrate systems, like m- and
p-tyramine, β-phenylethylamine,
p-octopamine, tryptamine, p-synephrine and N,N-dimethyltryptamine
[2]. In invertebrates, they act directly as neurotransmitters
[3], while in mammalians their roles were mainly attributed to that of supporting catecholamine actions. Not by chance, trace amines are in different areas of mammalian brain and their distribution is strictly linked to the projection domains of noradrenaline, dopamine and serotonin pathways
[2]. However, at present they are considered to play important roles in both neurotransmission and neuromodulation. Moreover, ingestion of high amounts of trace amines can lead to toxicity, including indirect sympathomimetic effects resulting in amphetamine-like responses
[2][4], which will be discussed next.
Trace amines are obtained through direct and selective action of decarboxylases on amino acids by removing the carboxyl group. In β-phenylethylamine, tyramine and tryptamine synthesis, the aromatic L- amino acid decarboxylase (AADC) mediates L-phenylalanine, L-tyrosine, and L-tryptophan decarboxylation, whereas octopamine is formed after hydroxylation of
p-tyramine by dopamine β–decarboxylase. In addition, N-methylation represents another pathway by which tryptamine can be metabolized, being that the formation of N,N-dimethyltryptamine is catalyzed by indolethylamine–N–methyltransferase
[5].
2. Trace Amines
2.1. Sources
Trace amines are naturally present in the human organism, even if their endogenous production does not represent their primary source
[6]. In fact, fermented cheese, fermented cabbage, chocolate, raw and smoked meat, raw and fish products, alcoholic beverages, soybeans products are some of the foodstuffs where trace amines have been detected. Their presence and amounts in food varies, which can be affected by intrinsic and extrinsic factors, such as: availability of free amino acids; presence of microorganism with aminogenic capacity, namely decarboxylase-positive microorganisms, and overall conditions that allow microorganisms’ growth
[7].
Moreover, food matrixes own different profiles of trace amines, depending on free amino acid and protein amounts in the starting materials
[8]. They are usually present in foodstuffs following degradation of proteins and peptides, including proteolysis by microorganisms. Biogenic amines and consequentially trace amines’ production occurs in the presence of microorganisms able to decarboxylate amino acids through substrate-specific enzymes, as stated
[9][10]. Decarboxylases are a group of enzymes that belong to the pyridoxal-phosphate-dependent family, where pyridoxal-5′-phosphate represents the coenzyme. Decarboxylase-positive microorganisms are naturally present in food, being often added during the manufacturing processes. Thus, trace amines production depends on the microorganism strain and its’ habitat conditions
[11]. Not by chance, differences in the quantitative and qualitative profiles of trace amines have been observed concurrently with microbiological changes, as a result of the variable response of the decarboxylase activity to environmental factors
[12]. According to Gardini et al., temperature, pH and salt concentration represent the most relevant factors affecting biogenic amines quantity in foods, but storage and production processes may also modulate their profile. These conditions act synergically augmenting or reducing the presence of trace amines. Tyramine, and other biogenic amines, are usually predominant during ripening, even if a decrease in their maximum concentration occurs in conformity with variations in the microbiota
[13]. Several amines, such as histamine, tyramine and phenylethylamine have been found in different types of ripened cheese, where they are mainly produced and accumulated by lactic acid bacteria or by microorganisms with decarboxylase activity (e.g.,
Bacillus,
Pseudomonas,
Escherichia,
Enterobacter,
Salmonella,
Shigella,
Staphylococcus,
Streptococcus,
Lactobacillus,
Enterococcus,
Lactococcus, and
Leuconostoc)
[6][12]. Even so, many elements promote or inhibit microbial activity during spontaneous fermentation: the use of food additives, or other synthetic and natural compounds can in fact inhibit the formation of amines
[14]. Caraway and onion reduce the amount of the biogenic amine in sauerkraut, depending also on the temperature during its preparation, for instance. The salt concentration is another impacting factor that contributes to a reduction in trace amines in foods, as the activity of decarboxylating microbiota is inhibited by the addition of salt depending on bacteria species present
[12]. The following table shows some sources and amounts of trace amines in food (
Table 1).
Table 1.
Major trace amines tyramine (TYR), β-phenylethylamine (PEA), and tryptamine (TRYP) found in food.