Simultaneous Mass Spectrometric Analysis of Methylated and Ethylated Peptides in Human Hemoglobin: Correlation with...

Alkylating agents contained in cigarettes smoke might be related to cancer development. Post-translational protein methylation and ethylation may cause alteration of protein functions. Human hemoglobin (Hb) has been a target for molecular dosimetry because of its easy accessibility. The goal of this study is to investigate the relationship between the levels of methylation and ethylation at specific sites of Hb with smoking. Because of the low extent of modification of Hb isolated from blood, the methylation and ethylation sites were identified in Hb incubated with a methylating agent (methyl methanesulfonate, MMS) and ethylating agent (ethyl methanesulfonate, EMS), respectively, by accurate mass measurements. After trypsin digestion, the modification sites were identified by nanoflow LC-nanospray ionization coupled with high-resolution mass spectrometry. The selected reaction monitoring mode was used to quantify the relative extent of methylation and ethylation in human Hb incubated with MMS and EMS, respectively. Methylation occurred at 9 sites, including ¹V, ²⁰H, ⁵⁰H, ⁷²H of α-globin and ¹V, ²⁶E, ⁶⁶K, ⁷⁷H, ⁹³C of β-globin. Ethylation was detected at 11 sites, including ¹V, ¹⁶K, ⁵⁰H, ⁷²H, ⁸⁷H of α-globin and ¹V, ¹⁷K, ⁶⁶K, ⁷⁷H, ⁹²H, ⁹³C of β-globin. The relative extents of methylation and ethylation were measured in blood samples from 13 smokers and 13 nonsmokers. No statistically significant difference was found in the methylated peptides. On the other hand, the extents of ethylation at α-terminal Val, α-His-50, α-His-87, β-terminal Val, β-His-77, and β-Cys-93 in Hb were significantly higher in smokers than in nonsmokers (p < 0.05). Furthermore, the relative extents of ethylation at these sites were statistically significantly correlated with the number of cigarettes smoked per day. Therefore, this assay, which requires as little as one drop of blood, should be helpful in measuring Hb ethylation as a potential biomarker for assessing the exposure to cigarette smoking.