Effect of substrate structure on activity of pigeon liver acetyl transferase.

The acetylation of aromatic amines by enzymes extracted from pigeon liver has been studied intensively, and the over-all process has been resolved into two clearly distinct steps. The pigeon liver extract that can acetylate sulfanilamide by utilizing adenosine triphosphate, coenzyme ,2, and acetate was treated with acetone to yield first, a fraction that produced acetyl-CoA and second, a fraction that transferred the acetyl from acetylCoA to sulfanilamide (1). For the acetyl transferase, a convenient spectrophotometric assay was obtained by replacing sulfanilamide with p-nitroaniline (2). It was further shown that the acetyl donor need not be acetyl-CoA, but could be an acetylated aromatic amine, and that the reaction was independent of CoA (3) ; actually, the addition of reduced CoAl inhibited the transfer reaction (2). As expected, the reaction between acetylCoA and p-nitroaniline proceeded to completion (2), but the transfer of acetyl from one aromatic amine to another, p-(p-acetylaminophenylazo)benzenesulfonate to sulfanilamide, was readily reversible and appeared to have an equilibrium constant near unity (3). In a previous paper (4), the acetyl transfer reaction from pnitroacetanilide to aniline was used to investigate the means by which amethopterin inhibition of sulfanilamide acetylation occurs. In the course of these studies the reversal of the reaction (acetanilide + p-nitroaniline) was sought but could not be detected. This indication of an irreversible reaction was in contrast to the previous report of the reversible transacetylation between aromatic amines (3). The present report is concerned with this apparent discrepancy and its resolution in terms of the electronegativity of the para substituent of the acetyl donor molecule.