The Myeloablative Drug Busulfan Converts Cysteine to Dehydroalanine and Lanthionine in Redoxins

The myeloablative agent busulfan (1,4-butanediol dimethanesulfonate) is an old drug that is used routinely to eliminate cancerous bone marrow prior to hematopoietic stem cell transplant. The myeloablative activity and systemic toxicity of busulfan have been ascribed to its ability to cross-link DNA. In contrast, here we demonstrate that incubation of busulfan with the thiol redox proteins glutaredoxin or thioredoxin at pH 7.4 and 37 °C results in the formation of putative S-tetrahydrothiophenium adducts at their catalytic Cys residues, followed by β-elimination to yield dehydroalanine. Both proteins contain a second Cys, in their catalytic C-X-X-C motif, which reacts with the dehydroalanine, the initial Cys adduct with busulfan, or the S-tetrahydrothiophenium, to form novel intramolecular cross-links. The reactivity of the dehydroalanine (DHA) formed is further demonstrated by adduction with glutathione to yield a lanthionine and by a novel reaction with the reducing agent tris(2-carboxyethyl)phosphine (TCEP), which yields a phosphine adduct via Michael addition to the DHA. Formation of a second quaternary organophosphonium salt via nucleophilic substitution with TCEP on the initial busulfan-protein adduct or on the THT(+)-Redoxin species is also observed. These results reveal a rich potential for reactions of busulfan with proteins in vitro, and likely in vivo. It is striking that several of the chemically altered protein products retain none of the atoms of busulfan, in contrast to typical drug-protein adducts or traditional protein modification reagents. In particular, the ability of a clinically used drug to convert Cys to dehydrolanine in intact proteins, and its subsequent reaction with biological thiols, is unprecedented.