Rat liver 4S-benzo[a]pyrene-binding protein is distinct from glycine N-methyltransferase

Rat liver 4S-benzo[a]pyrene-binding protein (BAP) was reported to be identical to the subunit (Mr 32,500) of the tetrameric glycine N-methyltransferase (GNMT). We have reevaluated this study. When a liver cytosol was subjected to Sephacryl S-200 gel permeation chromatography, enzyme activity as well as cross-reactivity with anti-GNMT antibody were found only at the elution position of the purified GNMT. Chromatograph of the cytosol pretreated with [3H]benzo[a]pyrene showed two peaks in the void volume and at the position of an approximate Mr of 40,000. The latter, corresponding to the 4 S BAP, did not cross-react with the anti-GNMT antibody. An extract of nuclei in which GNMT was proposed to act as a mediator of cytochrome P4501A1 gene expression contained the tetrameric GNMT but no binding activity. The lung, in which no GNMT mRNA occurred, had the 4 S BAP. Extracts of Escherichia coli and COS-7 cells expressing large amounts of GNMT showed no 4 S BAP. These findings suggest that the 4 S BAP is distinct from the subunit of GNMT.