Catalytic effect of serum albumin on the o-rearrangement of N-sulfooxy-2-acetylaminofluorene, a potential hepatocarcinogen in the rat, to nonmutagenic sulfuric acid esters of o-amidofluorenols

Bovine serum albumin (BSA) catalyzes the o-rearrangement of the reactive electrophile, N-sulfooxy-2-acetylaminofluorene (NSF), a potential ultimate hepatocarcinogen in the rat, to the nonmutagenic sulfuric acid esters of 1- and 3-hydroxy-2-acetylaminofluorene. Conversion of NSF was proportional to BSA concentrations ranging from 0.25 to approximately 4 mg BSA/ml incubation mixture. At concentrations greater than or equal to 5 mg BSA/ml, approximately 90% of NSF was converted to the sulfuric acid esters of the o-amidofluorenols. Human serum albumin (HSA) likewise catalyzed the o-rearrangement of NSF. However, the catalytic activity of HSA was only approximately 50% of the activity of BSA. The catalytic effect of BSA was abolished by heat denaturation. However, it was not changed by dialysis or by anion exchange chromatography. These observations indicated that the catalytic effect requires intactness of the tertiary structure of BSA and is not due to a contaminant(s) of low or high molecular weight. There were no differences in the catalytic activity of three separate fractions of chromatographed BSA, suggesting that the catalytic activity is associated with the entire BSA molecule. In contrast to serum albumin, gamma-globulin (bovine or human) did not catalyze the o-rearrangement of NSF. The solvolytic degradation of NSF to 4-hydroxy-2-acetylaminofluorene, a major reaction in the absence of BSA, occurred only to a minor extent in the presence of BSA. These data indicated that the BSA-catalyzed o-rearrangement determines the rates of concurrent reactions involved in the degradation of NSF. BSA and HSA did not catalyze the o-rearrangement of N-acetoxy-2-acetylaminofluorene (N-OAC-2-AAF), the acetate ester of N-hydroxy-2-acetylaminofluorene (N-OH-2-AAF), to the acetic acid esters of the o-amidofluorenols. These findings suggest that the albumin-catalyzed o-rearrangement occurs preferentially with esters of fluorenylhydroxamic acids that readily ionize in aqueous media.