Western blot analysis of 3T3-L1 adipocyte proteins using an anti-C/EBP alpha antibody detected a 24 kD polypeptide in addition to the expected 42 and 30 kD isoforms of C/EBP alpha. Mass spectrometric sequencing of the protein Following its purification by HPLC and preparative 2D gel electrophoresis identified it its glutathione S-transferase zeta/maleylacetoacetate isomerase (GST zeta/MAAI). Expression of GST zeta/MAAI mRNA and protein was included during the terminal phase of adipogenesis in 3T3-L1 preadipocytes. Ectopic expression of PPAR gamma 2 in NIH-3T3 fibroblasts exposed to insulin and troglitazone-induced perilipin production, but was incapable of activating GST zeta/MAAI Unless C/EBP alpha was also expressed. Similarly, ectopic expression of C/EBP alpha in PPAR gamma +/- or PPAR gamma -/- MEFs demonstrated that the C/EBP alpha-dependent induction of GST zeta/MAAI production was dependent on expression of endogenous PPAR gamma. These data suggest a role for GST zeta/MAAI in mature adipocytes that may be responsive to the thiazolidinedione class of insulin sensitizing PPAR gamma ligands. (c) 2005 Elsevier Inc. All rights reserved.