We recently identified a microbial conversion of L-ascorbic acid (AsA) to L-erythroascorbic acid (eAsA), a five-carbon analog of AsA. In this paper, we show that ubiquitin plays a crucial role in this process. Based on an assay that determined AsA decomposition, we purified proteins that had N-terminal amino acid sequences identical to that of yeast ubiquitin. Purified ubiquitin facilitated decompositions of AsA and dehydro-AsA, accompanying a partial conversion to eAsA through Cl-elimination. Acetylation or limited hydrolysis of ubiquitin abolished its activity. A mutant ubiquitin, with Lys(6) replaced by Arg, completely lost activity, whereas a mutant, with six other Lys residues (positions at 11, 27, 29, 33, 48 and 63) substituted by Arg, retained activity. Thus, Lys(6), Which locates in close proximity to H is crucial for ubiquitin activity in the AsA conversion to eAsA. (C) 2009 Elsevier Inc. All rights reserved.