Technical note: quantification of plasma 1- and 3-methylhistidine in dairy cows by high-performance liquid chromatography-tandem mass spectrometry.

To improve monitoring of protein mobilization in dairy cows, we developed and evaluated a method to quantify 1-methylhistidine and 3-methylhistidine in plasma by HPLC-tandem mass spectrometry. The analytical method described is (1) sensitive: both histidine derivates can be detected in the picomole range; (2) accurate: intra- and interassay coefficients of variation were < 5% for all standard solutions of 1-methylhistidine and 3-methylhistidine measured (31 to 500 pmol); (3) specific: 1-methylhistidine is clearly separated from 3-methyl-histidine in plasma samples from dairy cows; and (4) flexible: can be easily adapted to measure other amino acids or compounds containing a primary amine. 1-Methylhistidine is present in plasma of dairy cows at concentrations of 5.0 ± 1.7 μM, similar to concentrations of 3-methylhistidine (4.4 ± 2.4 μM). Analytical separation of both histidine metabolites is essential when plasma 3-methylhistidine is used as indicator for muscle breakdown in dairy cows. Specific quantification of the concentration of 3-methylhistidine in bovine plasma samples by HPLC tandem mass spectrometry can improve monitoring of protein mobilization in dairy cows.