We describe for the first time a naturally occurring lysine modification that is converted to methyllysine by reduction with sodium borohydride. This modification is 1.7 times as abundant in soluble proteins from human substantia nigra pars compacta as in proteins from other brain regions, possibly as a result of elevated oxidative stress in the nigra. Proteins from cultured PC12 cells exposed to oxidative stress conditions also contain elevated levels of this lysine modification. The abundance of the naturally occurring modification is roughly 0.08 nmoles/mg protein in either unstressed brain or PC12 cells. Modification levels remain stable in isolated proteins incubated for 2 h at 37°C in pH 7 buffer. We propose that the endogenous modification is the lysine Schiff base, e-N-methylenelysine, and that lysine modifications may result from a reaction with formaldehyde in vivo. Rat brain contains 60 nmoles/g wet weight of formaldehyde, which probably includes both free and reversibly bound forms. Adding 35 lM HCHO to PC12 cell growth medium introduces methylenelysine modifications in cell proteins and impairs cell viability. The existence of this posttranslational modification suggests new mechanisms of oxidative stress that may contribute to tissue degeneration, including loss of nigral dopamine neurons during normal aging and in Parkinson's disease.