The isolation and characterization of 3-(2-carboxyethyl)cytosine following in vitro reaction of beta-propiolactone with calf thymus DNA.

The new adduct 3-(2-carboxyethyl)cytosine (3-CEC) was isolated following in vitro reaction of the carcinogen beta-propiolactone (BPL) with calf thymus DNA. The structure of 3-CEC was confirmed by synthesis from BPL and dCyd. Reaction of BPL with cCyd (pH 7.0-7.5, 37 degrees C) gave 3-(2-carboxyethyl)deoxycytidine (3-CEdCyd) (9% yield) and 3,N4-bis(2-carboxyethyl)deoxycytidine (3,N4-BCEdCyd) (0.6% yield). 3-CEdCyd and 3,N4-BCEdCyd were hydrolyzed (1.5 N HCl, 100 degrees C, 2 h) to 3-CEC and 3,N4-bis(2-carboxyethyl)cytosine (3,N4-BCEC), respectively. The structure of 3-CEC was assigned on the basis of UV and NMR spectra and the electron impact (EI) mass spectra of 3-CEC and a tri-trimethylsilyl (TMS) derivative of 3 CEC as well as deuterated (d27) tri-TMS derivative of 3-CEC. The structure of 3,N4-BCEC was assigned on the basis of UV spectra and the EI mass spectra of a tri-TMS derivative. Ei and isobutane chemical ionization mass spectra of 3-methylcytosine (3-MeCyt) and a di-TMs derivative of 3-MeCyt were obtained and were helpful in deducing the structures of 3-CEC and 3,N4-BCEC. This is the first report of the alkylation by BPL of an exocyclic atom on a base in DNA. Compound 3,N4-BCEC was not detected in BPL-reacted calf thymus DNA. The relative amounts of 1-(2-carboxyethyl)-adenine (1-CEA), 7-(2-carboxyethyl)guanine (7-CEG), 3-(2-carboxyethyl)-thymine (3-CET) and 3-CEC isolated from BPL-reacted DNA following perchloric acid hydrolysis were 0.23, 1.00, 0.39 and 0.41 respectively, when the alkylation reaction was conducted in phosphate buffer at 0-5 degrees C and pH 7.5 and 0.10, 1.00, 0.29 and 0.28 respectively when the reaction was conducted in H2O at 37b degrees C and pH 7.0-7.5.