HUMAN LIVER-S-9 METABOLIC-ACTIVATION - PROFICIENCY IN CYTOGENETIC ASSAYS AND COMPARISON WITH PHENOBARBITAL BETA-NAPHTHOFLAVONE OR AROCLOR-1254 INDUCED RAT-S-9/

Induced rat liver S-9 is routinely used for metabolic activation in cy togenetic assays. When a compound gives a positive test result only wi th rat S-9, the significance for humans should be assessed. To evaluat e the use of human S-9, we used sister-chromatid exchanges (SCEs) and chromosome aberrations (Abs) in Chinese hamster ovary cells to test fi ve pro-mutagens, each preferentially activated by a different family o f P-450: benzo(a)pyrene (BP), dimethylnitrosamine (DMN), diethylnitros amine (DEN), aflatoxin B-1 (AFB), and 2-acetylaminofluorene (2-AAF). W e tested two human S-9 preparations, one from a single liver and a sec ond pooled from two livers known to have good activity for several P-4 50s. Concentrations and ratios of NADP and isocitrate were adjusted to optimize NADPH generation by the S-9. Abs were scored 20 hr, and SCEs 29-45 hr, after the beginning of a 3 hr treatment. P-450 enzyme activ ities were generally higher in rat than human S-9. With the single-liv er human S-9, increases in SCEs were seen with all chemicals; with bot h human S-9s, increases in Abs were seen with all chemicals except BP. (The level of P-450 1A1, required For BP activation, is very low in h uman liver.) Compared with mt S-9, generally higher concentrations of human S-9 and of promutagens were required to see positive results. Ho wever, human S-9 effectively activated 2-AAF, whereas neither of the t wo types of rat S-9 produced Abs with 2-AAF. We also compared rat S-9s induced with Aroclor 1254 or phenobarbital/beta-naphthoflavone (PB/be ta NF). Although there were some differences in P-450 enzyme activitie s, these did not translate into differences in Abs induction. At low d oses of AFB and of BP, PB/beta NF induced S-9 appeared more effective than Aroclor 1254 induced S-9. (C) 1996 Wiley-Liss, Inc.