| Abstract: | We have reported previously that phenylarsine oxide (PAO) blocks insulin-stimulated glucose transport in 3T3-L1 adipocytes (Frost, S. C., and Lane, M. D. (1985) J. Biol. Chem. 260, 2646-2652). As shown in the present study, the locus of inhibition is post-receptor. Insulin stimulated the extent of receptor autophosphorylation in solution and in the intact cell by approximately 4-fold. PAO had no effect on this activity. Using reduced and carboxamidomethylated lysozyme as a substrate for the tyrosine-specific receptor, insulin stimulated the rate of receptor kinase-catalyzed substrate phosphorylation by 2-fold; PAO had no effect on this stimulation. However, the insulin-stimulated, serine-specific phosphorylation of two endogenous phosphoproteins (pp24 and pp240) in the intact cell was blocked by 25 microM PAO. These complementary in situ and in vitro studies demonstrate that the inhibition by PAO must be distal to the insulin receptor's protein tyrosine kinase activity. |
