TRYPSIN-MN2-RESISTANT FORM OF TYPE-1 PROTEIN PHOSPHATASE IN HUMAN MUSCLE()

Reduced type 1 protein phosphatase (PP-1) activity in human muscle ext racts may contribute to the reduced insulin-stimulated glycogen syntha se activity associated with insulin resistance for glucose disposal in humans. Because inactive forms of PP-1 can be activated with trypsin plus Mn2+, these reagents were used to compare the PP-1 activities in skeletal muscle extracts before and after separation into cytosolic an d glycogen microsomal (GM) fractions. PP-1 activities were reduced in the GM fraction from insulin-resistant subjects (54 +/- 2 vs. 61 +/- 1 , P < 0.01) but, in contrast to our previously published results, were elevated in the extract (33 +/- 6 vs. 18 +/- 3, P < 0.05). Recombinat ion of the cytosol and GM fractions (reconstituted extract) demonstrat ed that the low extract PP-1 activities could only be regenerated when the GM fraction from insulin-sensitive subjects was recombined with c ytosol from either group. The results indicate that the elevated PP-1 activity observed in extracts of insulin-resistant compared with insul in-sensitive subjects is caused by an inhibitor of extract PP-1 activi ty that sediments with the GM pellet and is more active in the insulin -sensitive subjects.