A Novel Enzymatic Method for Measuring Mizoribine 5′-Monophosphate Levels in Serum

Inosine 5′-monophosphate dehydrogenases (IMPDH) from Oceanobacillus iheyensis and Bacillus subtilis were biochemically characterized with the aim of establishing a mizoribine 5′-monophosphate (MZR-P) assay. MZR-P is the active metabolite of mizoribine, which has been successfully used as an immunosuppressive agent. A sensitive method for measuring the MZR-P concentration in serum, based on its ability to inhibit IMPDH, was developed. The method was applied to an automatic clinical analyzer, enabling measurement of MZR-P levels in a set of serum samples. MZR-P at concentrations of 0.1–13.6 (μg/ml (0.3–40 μM) in the samples could be measured with 0.7% within-run coefficients of variation.