CHARACTERIZATION OF RAT HEPATIC ACETYLTRANSFERASE

Rat liver cytosol is capable of N-acetylation (NAT) of arylamines, O-a cetylation (OAT) of arylhydroxylamines, and N,O-acetyltransfer (AHAT) of arylhydroxamic acids. Physical, enzymatic, and immunochemical techn iques now support the conclusion that a single 32 kDa protein accounts for all of these activities. Of the five immunoglobulin (IgG(1)) mous e monoclonal antibodies (mAb) produced against this protein, each affe cted one or more of these acetylation activities. When mixed with rat hepatic cytosol and then chromatographed on a gel filtration column, m Abs 1F2 and 5F8 increased the apparent size of all enzymes capable of acetylation from 32 kDa to the exclusion volume. Each of the mAbs reac ted with only a single 32 kDa protein on SDS-PAGE/Western blots, regar dless 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 AHA T activity, but it does not result in changes in either molecular weig ht 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 wit hout change in molecular weight. Although NAT activity is retained in acetyltransferase (AT) that has a minimally modified pl, further incre ases in pl result in total loss of enzyme activity. The differential e ffects 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, conseq uently, provide insight as to why the abilities of ATs from different species exhibit such dissimilar potentials for the activation of aroma tic amines by OAT and AHAT.