PHENYLARSINE OXIDE AND VANADATE - APPARENT PARADOX OF INHIBITION OF PROTEIN PHOSPHOTYROSINE PHOSPHATASES IN RAT ADIPOCYTES

Vanadate mimics, whereas phenylarsine oxide (PAO) antagonizes, the eff ects of insulin in rat adipocytes. Both vanadate and PAO are documente d inhibitors of protein-phosphotyrosine phosphatases. The relationship between the inhibition of 'inhibitory' PTPase and 'stimulatory' PTPas e has been studied here in primary rat adipocytes. Low concentrations of PAO (IC50 = 0.6-2.0 mu M) blocked the stimulating effects of insuli n, vanadate and pervanadate on hexose uptake and glucose metabolism. I nhibition of isoproterenol-mediating lipolysis by vanadate and insulin was not blocked by PAO. The activating effects of okadaic acid on hex ose uptake and glucose metabolism, which occur at points downstream to tyrosine phosphorylation, were also not blocked by PAO. Subsequent st udies suggested that the PAO-sensitive PTPase comprises a minute fract ion of the total adipocytic PTPase activity. To identify its location we applied procedures involving fractionations and activation of non-r eceptor adipocytic protein tyrosine kinase by PAO and vanadate in cell free assays. We found that the 'inhibitory' PTPase is exclusively ass ociated with the membrane fraction whereas the 'stimulatory' PTPases a re present in both the cytosolic and plasma membrane compartments. We next searched for markers, possibly associated with PAO-dependent dese nsitization and found that several proteins became phosphorylated on t yrosine moieties in the supernatant of PAO but not in vanadate pretrea ted adipocytes. In summary, we propose the presence of a minute, plasm a membrane associated PTPase in primary rat adipocytes, inhibition of which arrests the activation of glucose metabolism. In contrast, inhib ition of all the other cellular adipose PTPases, ultimately activates rather than inhibits these same bioeffects.