Development of a novel L-histidine assay method using histamine dehydrogenase and a stable mutant of histidine decarboxylase.

l-Histidine analysis is essential in physiological research and clinical applications because l-histidine concentrations in biofluids are associated with various diseases. However, an enzymatic method for l-histidine quantitation has not yet been established. Here, we describe a novel l-histidine quantitation assay using a combination of histidine decarboxylase (HDC) and histamine dehydrogenase (HDH) enzymes. Wild-type HDC is unstable and completely lost its activity within 50 days of storage at 4 °C in solution. We rationally designed a HDC C57S mutant with markedly improved stability (storage at 4 °C for over 200 days) without altering the enzyme's substrate specificity. Together with HDH, the HDC C57S mutant was applied to quantify l-histidine concentrations in human plasma. The assay showed high precision (<2.0% inter-assay variation) and high accuracy (<5.8% deviation from the results of LC/MS).