Lysosomal prenylcysteine lyase is a FAD-dependent thioether oxidase

Prenylated proteins contain either a 15-carbon farnesyl or a 20-carbon gera nylgeranyl isoprenoid covalently attached via a thioether bond to a cystein e residue at or near their C terminus. As prenylated proteins comprise up t o 2% of the total protein in eukaryotic cells, and the thioether bond is a stable modification, their degradation raises a metabolic challenge to cell s. A lysosomal enzyme termed prenylcysteine lyase has been identified that cleaves prenylcysteines to cysteine and an unidentified isoprenoid product. Here we show that the isoprenoid product of prenylcysteine lyase is the C- l aldehyde of the isoprenoid moiety (farnesal in the case of C-15). The enz yme requires molecular oxygen as a cosubstrate and utilizes a noncovalently bound flavin cofactor in an NAD(P)H-independent manner. Additionally, a st oichiometric amount of hydrogen peroxide is produced during the reaction. T hese surprising findings indicate that prenylcysteine lyase utilizes a nove l oxidative mechanism to cleave thioether bonds and provide insight into th e unique role this enzyme plays in the cellular metabolism of prenylcystein es.