Synthesis of 5,5,6,6‐D4‐L‐lysine‐aflatoxin Bl for use as a mass spectrometric internal standard

Human exposure to the hepatocarcinogenic mycotoxin aflatoxin B_l results in modification of serum albumin lysine ε‐amino residues to form lysine‐aflatoxin adducts. A perdeuterated reference standard is now required to quantitatively measure this adduct in epidemiologic studies of liver cancer using isotopic dilution mass spectrometry. A convenient method for the preparation of D4‐L ‐lysine‐AFB_l using commercially available 5,5,6,6‐D4‐l ‐lysine is demonstrated for the first time. The application of two standard α‐amino protection methods is also reported that simplifies the production of natural isotopic abundance lysine‐AFB_l over the currently used method employing N_α‐acetyl‐l ‐lysine. t‐Boc‐N_α‐lysine was used to prepare lysine‐AFB_l; however, a preferred method for directing reaction of AFB_l‐dialdehyde to the ε‐amino group of 5,5,6,6‐D4‐l ‐lysine utilized cupric ions that were spontaneously removed during the reverse phase HPLC purification of D4‐lysine‐AFB_l using 1% HOAc. This strategy eliminates the need to otherwise synthesize and purify t‐Boc‐N_α‐ or N_α‐acetyl‐5,5,6,6‐D4‐lysine and then TFA or enzymatically deprotect overnight to obtain the target compound. Copyright © 2004 John Wiley & Sons, Ltd.