Quantitative analysis of N-terminal valine peptide adducts specific for 1,2-epoxy-3-butene

Abstract

Butadiene (BD) metabolism shows gender, species and concentration dependency, making the extrapolation of animal results to humans complex. BD is metabolized mainly by cytochrome P450 2E1 to three epoxides, 1,2-epoxy-3-butene (EB), 1,2;3,4-diepoxybutane (DEB) and 1,2-epoxy-butanediol (EB-diol). For accurate risk assessment it is important to elucidate species differences in the internal formation of the individual epoxides in order to assign the relative risks associated with their different mutagenic potencies. Analysis of N-terminal globin adducts is a common approach for monitoring the internal formation of BD derived epoxides. Our long term strategy is to develop an LC-MS/MS method for simultaneous detection of all three BD hemoglobin adducts. This approach is modeled after the recently reported immunoaffinity LC-MS/MS method for the cyclic N,N-(2,3-dihydroxy-1,4-butadyil)-valine (pyr-Val, derived from DEB). We report herein the analysis of the EB-derived 2-hydroxyl-3-butenyl-valine peptide (HB-Val). The procedure utilizes trypsin hydrolysis of globin and immunoaffinity (IA) purification of alkylated heptapeptides. Quantitation is based on LC-MS/MS monitoring of the transition from the singly charged molecular ion of HB-Val (1-7) to the a(1) fragment. Human HB-Val (1-11) was synthesized and used for antibody production. As internal standard, the labeled rat-[(13)C(5)(15)N]-Val (1-11) was prepared through direct alkylation of the corresponding peptide with EB. Standards were characterized and quantified by LC-MS/MS and LC-UV. The method was validated with different amounts of human HB-Val standard. The recovery was >75% and coefficient of variation <25%. The LOQ was set to 100 fmol/injection. For a proof of principal experiment, globin samples from male and female rats exposed to 1000 ppm BD for 90 days were analyzed. The amounts of HB-Val present were 268.2+/-56 and 350+/-70 pmol/g (mean+/-S.D.) for males and females, respectively. No HB-Val was detected in controls. These data are much lower compared to previously reported values measured by GC-MS/MS. The difference may be due higher specificity of the LC-MS/MS method to the N-terminal peptide from the alpha-chain versus derivatization of both alpha- and beta-chain by Edman degradation, and possible instability of HB-Val adducts during long term storage (about 10 years) between the analyses. These differences will be resolved by examining recently collected samples, using the same internal standard for parallel analysis by GC-MS/MS and LC-MS/MS. Based on our experience with pyr-Val adduct assay we anticipate that this assay will be suitable for evaluation of HB-Val in multiple species.

Fig. 1

Characterization of rat [¹³C₅¹⁵N]HB-Val (1–11) and (1–7) internal standard peptide. (A) Full scan (m/z 100–1500) of rat [¹³C₅¹⁵N]HB-Val (1–11) and (B) MS/MS spectrum of rat [¹³C₅¹⁵N]HB-Val (1–7)

Fig. 2

Calibration curve for HB-Val.

Fig. 3

Ion-chromatogram of HB-Val peptide from a male rat exposed to 1000 ppm BD for 90 days. Shown are the transitions of the analyte (m/z 817 → 142) and internal standard peptide (m/z 823 → 147).

Fig. 4

Comparison of HB-Val in male and female rats exposed to 1000 ppm BD for 90 days (mean ± S.D., n = 6).

Scheme 1

BD metabolism and formation of globin adducts.