A Spectrophotometric Assay for Thiogalactoside Transacetylase

was first detected by Zabin, Kepes, and Monod in extracts of Escherichia co& (I), and has recently been crystallized (2). The enzyme is considered to be a product of the lactose operon. During induction of P-galactosidase, transacetylase activity increases proportionately. Furthermore, in certain galactoside permeaseless mutants (y-), low levels of transacetylase are found. Its physiological function and importance are unknown, alt’hough it has been suggested that it may be related to galactoside transport (1). Investigations of transacetylase have been limited by the relatively cumbersome assays presently in use, which are based either on the formation of an acetylhydroxamate from the acetylated thiogalactoside (3) or on the detection of radioactive acetylated thiogalactosides (4). The present communication presents a rapid spectrophotometric assay which is about 100 times more sensitive than the hydroxamate assay, and which can be used with crude bacterial extracts. This new method is based on a disulfide interchange reaction between the CoA liberated from acetyl-CoA and the dithiobis(2nitrobenzoic acid) reagent described by Ellman (5) (Reaction 2).