Insulin-like Effects of Trypsin on Fat Cells LOCALIZATION OF THE METABOLIC STEPS Ai’?D THE CELLULAR SITE AFFECTED BY THE ENZYME*

The mechanism of the insulin-like action of trypsin was studied. When fat cells were exposed to trypsin (1 mg per ml) for 15 set, glucose metabolism in the cells was stimulated. This stimulation (like that by insulin) was inhibited by phloretin or 3-O-methyl-n-glucose. By the above treatment with trypsin, cellular lipolysis induced by epinephrine was blocked to a large extent, but that induced by caffeine was only partially inhibited. These effects of trypsin on lipolysis were similar to those of insulin at a low concentration (20 to 30 microunits per ml). Trypsin (like insulin) did not inhibit lipolysis induced by N6, iY’-dibutyryl adenosine 3’, 5’-monophosphate. When fat cells treated with trypsin were incubated with buffer for a few hours after inactivation of the enzyme, the cells had recovered from the insulin-like effects of the enzyme. The recovery was not significantly inhibited by cycloheximide. The “recovered” cells were less sensitive to both insulin and trypsin as compared with untreated fat cells. This decrease in sensitivity was partially prevented when a large dose of insulin was added to the incubation medium prior to the trypsin treatment. These data were consistent with an assumption that trypsin (a) interacts initially with the insulin receptor site of fat cells, (b) activates the receptor to produce the metabolic responses characteristic of insulin, and (c) eventually modifies the receptor to render the cells less sensitive than normal to both insulin and trypsin.