O-Methylation of 3H-norepinephrine by epididymal adipose tissue

The presence of catechol-O-methyl transferase (COMT) activity in rat epididymal adipose tissue preparations was studied in vitro with ³H-norepinephrine as substrate. A radioactive metabolite which behaved chromatographically like normetanephrine (NM) was isolated from the incubation medium. It was possible to inhibit the formation of this metabolite by the addition of pyrogallol or of nonradioactive NM to the incubation medium. Whole epididymal adipose tissue had greater COMT activity than did isolated adipocytes containing an equivalent amount of triglyceride. Centrifugation of adipocyte homogenates revealed that twice as much COMT activity was in the 30,000 g particulate as in the supernatant fractions. This enzyme activity was further found to reside in preparations of adipocyte plasma membranes. These observations suggest that adipocyte COMT exists partially in a particle-bound form, probably associated with the cell membrane.