Histidinemia is an autosomal recessive metabolic disorder that is characterized by increased levels of histidine in biofluids such as the blood, urine, and cerebrospinal fluid of pediatric patients. The affected enzyme is histidase, which is responsible for the conversion of histidine to urocanic acid. Alternative biochemical mechanisms that include the acetylation, methylation, and transamination or amino group transfer of histidine are increased in patients with histidinemia. Also increased in patients is the decarboxylation of histidine by histidine decarboxylase to form histamine.

Due to its role as a neurotransmitter and immunomodulator, histamine metabolism in histidinemic patients was studied by measuring the urinary levels of histamine and its metabolites. The results of the experiments showed that urinary excretions of histamine metabolites, N-τ-methylhistamine, imidazole acetic acid, and its conjugates were higher in patients with histidinemia than in healthy controls. Urinary excretion of 3-methylhistidine, a biomarker of histidinemia, also showed a close positive correlation with the urinary excretion of histidine.

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