Characterization of rat hepatic acetyltransferase.

Rat liver cytosol is capable of N-acetylation (NAT) of arylamines, O-acetylation (OAT) of arylhydroxylamines, and N,O-acetyltransfer (AHAT) of arylhydroxamic acids. Physical, enzymatic, and immunochemical techniques now support the conclusion that a single 32 kDa protein accounts for all of these activities. Of the five immunoglobulin (IgG1) mouse monoclonal antibodies (mAb) produced against this protein, each affected one or more of these acetylation activities. When mixed with rat hepatic cytosol and then chromatographed on a gel filtration column, mAbs 1F2 and 5F8 increased the apparent size of all enzymes capable of acetylation from 32 kDa to the exclusion volume. Each of the mAbs reacted with only a single 32 kDa protein on SDS-PAGE/Western blots, regardless of the state of purity of the enzyme. This enzyme is unstable in low salt solutions, as reflected by a relative loss in NAT versus AHAT activity, but it does not result in changes in either molecular weight or isoelectric point (pl). A second form of instability is shown by the formation of more basic peptides with pls as high as 6, again without change in molecular weight. Although NAT activity is retained in acetyltransferase (AT) that has a minimally modified pl, further increases in pl result in total loss of enzyme activity. The differential effects of the mAbs on AT suggest that the ratios of NAT, OAT, and AHAT may be highly dependent on the conformation of the enzyme and, consequently, provide insight as to why the abilities of ATs from different species exhibit such dissimilar potentials for the activation of aromatic amines by OAT and AHAT.