Tryptophan metabolism and small fibre neuropathy: a correlation study

Small nerve fibres located in the epidermis sense pain. Dysfunction of these fibres decreases the pain threshold known as small fibre neuropathy. Diabetes mellitus is accompanied by metabolic changes other than glucose, synergistically eliciting small fibre neuropathy. These findings suggest that various metabolic changes may be involved in small fibre neuropathy. Herein, we explored the correlation between pain sensation and changes in plasma metabolites in healthy Japanese subjects. The pain threshold evaluated from the intraepidermal electrical stimulation was used to quantify pain sensation in a total of 1021 individuals in the 2017 Iwaki Health Promotion Project. Participants with a pain threshold evaluated from the intraepidermal electrical stimulation index <0.20 mA were categorized into the pain threshold evaluated from the intraepidermal electrical stimulation index-low group (n = 751); otherwise, they were categorized into the pain threshold evaluated from the intraepidermal electrical stimulation index-high group (n = 270). Metabolome analysis of plasma was conducted using capillary electrophoresis time-of-flight mass spectrometry. The metabolite set enrichment analysis revealed that the metabolism of tryptophan was significantly correlated with the pain threshold evaluated from the intraepidermal electrical stimulation index in all participants (P < 0.05). The normalized level of tryptophan was significantly decreased in participants with a high pain threshold evaluated from the intraepidermal electrical stimulation index. In addition to univariate linear regression analyses, the correlation between tryptophan concentration and the pain threshold evaluated from the intraepidermal electrical stimulation index remained significant after adjustment for multiple factors (β = -0.07615, P < 0.05). These findings indicate that specific metabolic changes are involved in the deterioration of pain thresholds. Here, we show that abnormal tryptophan metabolism is significantly correlated with an elevated pain threshold evaluated from the intraepidermal electrical stimulation index in the Japanese population. This correlation provides insight into the pathology and clinical application of small fibre neuropathy.

The region of alpha-lactalbumin recognized by GroEL

The binding constants between disulfide-intact or various disulfide-reduced bovine alpha-lactalbumins and an Escherichia coli chaperonin, GroEL, were determined by using the equilibrium dialysis method. The disulfide-intact and one-disulfide (Cys6-Cys120)-reduced alpha-lactalbumins were shown not to bind with GroEL both in the presence and absence of Ca2+. The two-disulfide (Cys6-Cys120 and Cys28-Cys111)-reduced alpha-lactalbumin, which has the native-like tertiary structure in its beta-domain region and an unfolded alpha-domain in the presence of Ca2+, showed considerable binding with GroEL. The binding free energy of the two-disulfide-reduced alpha-lactalbumin in the presence of Ca2+ is close to that of the molten globule state of disulfide-intact alpha-lactalbumin. This result suggests that GroEL binds to the unfolded alpha-domain of alpha-lactalbumin regardless of the conformation of the beta-domain. The fully disulfide-reduced and two-disulfide-reduced alpha-lactalbumins were found to bind more strongly with GroEL in the absence of Ca2+ than the two-disulfide-reduced alpha-lactalbumin in the presence of Ca2+, thus indicating that the unfolding of the beta-domain of alpha-lactalbumin leads to stronger interaction with GroEL.