A critical evaluation of the possible modulation of hepatic microsomal glucose-6-phosphatase activity by protein phosphorylation

Evidence is presented that incubation of intact microsomal vesicles with ATP and the catalytic subunit of cAMP-dependent protein kinase does not stimulate glucose-6-phosphatase. Our analyses show that commercial preparations of ATP, phosphoenol pyruvate, pyruvate kinase and protein kinase contain free phosphate that complicates interpretation of experimental data obtained using colorimetric assays of enzymic activity. The marked inhibition of enzymic activity by dithiothreitol, present in reconstituted preparations of protein kinase, also is a confounding factor. We recommend the use of glucose-6-phosphatase assays employing³²P-labelled substrate in future studies of this mechanism.