A mass spectrometric investigation on the possible role of tryptophan and 7-hydroxytryptophan in melanogenesis

Tryptophan in health and disease

Tryptophan (TRP), an essential amino acid in mammals, is involved in several physiological processes including neuronal function, immunity, and gut homeostasis. In humans, TRP is metabolized via the kynurenine and serotonin pathways, leading to the generation of biologically active compounds, such as serotonin, melatonin and niacin. In addition to endogenous TRP metabolism, resident gut microbiota also contributes to the production of specific TRP metabolites and indirectly influences host physiology. The variety of physiologic functions regulated by TRP reflects the complex pattern of diseases associated with altered homeostasis. Indeed, an imbalance in the synthesis of TRP metabolites has been associated with pathophysiologic mechanisms occurring in neurologic and psychiatric disorders, in chronic immune activation and in the immune escape of cancer. In this chapter, the role of TRP metabolism in health and disease is presented. Disorders involving the central nervous system, malignancy, inflammatory bowel and cardiovascular disease are discussed.

Polysaccharide-mediated synthesis of melanins from serotonin and other 5-hydroxy indoles

Aim:

As a continuation of our research on the melanin formation from catecholamines, we studied the polysaccharide-mediated oxidation of serotonin and other 5-hydroxy indoles into melanin-like materials. As for the catecholamines, we observed that many polysaccharides promote the oxidation of such compounds, particularly in the presence of Cu²⁺.

Methodology:

The reactions were monitored using reverse phase-HPLC and size-exclusion chromatography techniques. Melanin-like materials were purified through dialysis and characterized using UV-Vis and Fourier transform IR spectroscopic techniques.

Results:

One such material, synthesized from chondroitin sulfate type A and serotonin in the presence of Cu²⁺ was found to affect the release of IL-1β and IL-6 cytokines from immune cells.

The manuscript details our study of the formation of melanin-like pigments from serotonin in the presence of select polysaccharides. Melanin-like pigments have been observed in the brain and their presence is related to pathologies like Parkinson's disease. The melanin-like pigments observed in brain cells are derived from catecholamines like dopamine or norepinephrine. Our research suggests that another important brain compound, serotonin, may make contributions to the appearance of melanin-like pigments in brain tissues.

Redox mechanism of neurotoxicity by a serotonin-acrolein polymeric melanoid

Postoperative cognitive dysfunction may be associated with the toxic products of lipid peroxidation, such as the α,β-unsaturated aldehyde acrolein, which accumulates in aging. We previously identified an acrolein-mediated, serotonin-derived melanoid product, or SDM. This study further characterizes this putative novel neuromelanin, which is not made from catecholamines. In addition to its strong protein-binding properties, we observed that SDM binds Fe(2+) readily and exhibits complex redox characteristics. SDM may exist as a two-dimensional network of polymers that coalesce into larger entities exhibiting electroactive properties. These observations suggest that SDM may contribute to the decline in cognition due to focal degeneration from SDM-mediated free-radical production. We know that inhalational anesthetics sequester acrolein, which is toxic to neurons, and we propose that the local increase in acrolein depletes serotonin levels and enhances neuronal vulnerability through the production of neuromelanin-like structures, such as SDM.

Toxicity of a serotonin-derived neuromelanin

Postoperative Cognitive Dysfunction (POCD) is associated with increased mortality in the elderly and may occur from lipid peroxidation in aging. We previously showed that sevoflurane sequesters acrolein, which promotes the formation of a novel species of a putative neuromelanin. The current study examined the properties of this serotonin-derived melanoid (SDM). The interaction of SDM with unilamellar vesicles (ULVs) was examined using lipid membrane probes. Vesicle disruption was investigated by leakage of dye from calcein-loaded ULVs. We observed that SDM decreased diphenyl-hexatriene fluorescence anisotropy and increased the temperature-dependent change in anisotropy. SDM changed the absorbance of merocyanin-bound ULVs. SDM increased detergent-mediated calcein leakage. SDM structure was dramatically altered upon interaction with ULVs. We also observed that SDM enhanced detergent-mediated leakage of loaded ULVs, suggesting that SDM may be neurotoxic. We propose that inhalational agents, which sequester acrolein, may promote the production of certain species of neuromelanin that depletes local serotonin and enhances neuronal vulnerability.

Production of a novel neuromelanin at the sevoflurane-water interface

Postoperative cognitive dysfunction (POCD) occurs in the elderly following surgery that requires inhaled anesthetics. The molecular mechanism associated with this process is unknown. This study examined the possible role of serotonin, a neurotransmitter involved in cognition. We observed that sevoflurane, a common inhaled anesthetic, formed a separate phase in water similar to that of chloroform. Additionally, sevoflurane sequestered acrolein, which is a lipid peroxidation product associated with aging and is elevated in the elderly brain. The enhanced partitioning of acrolein increased the focal concentration and hence reactivity to serotonin which preferentially occurred at the sevoflurane-water interface. The resulting product exhibited unique properties similar to catecholamine-derived neuromelanin.

Nitration of tryptophan 372 in succinyl-CoA:3-ketoacid CoA transferase during aging in rat heart mitochondria

The main objective of this study was to test the hypothesis that in vivo post-translational modifications in proteins, induced by the endogenously generated reactive oxygen and nitrogen molecules, can alter protein function and thereby have an effect on metabolic pathways during the aging process. Succinyl-CoA:3-ketoacid coenzyme A transferase (SCOT), the mitochondrial enzyme involved in the breakdown of ketone bodies in the extrahepatic tissues, was identified in rat heart to undergo age-associated increase in a novel, nitro-hydroxy, addition to tryptophan 372, located in close proximity ( approximately 10 A) of the enzyme active site. Between 4 and 24 months of age, the molar content of nitration was more than doubled while specific enzyme activity increased significantly. The amount of SCOT protein, however, remained unchanged. In vitro treatment of heart mitochondrial soluble proteins with relatively low concentrations of peroxynitrite enhanced the nitration as well as specific activity of SCOT. Results of this study identify tryptophan to be a specific target of nitration in vivo, for the first time. We hypothesize that increases in tryptophan nitration of SCOT and catalytic activity constitute a plausible mechanism for the age-related metabolic shift toward enhanced ketone body consumption as an alternative source of energy supply in the heart.