Characterization of thyroidal membrane-bound Mg-adenosinetriphosphatase activated by trypsin or poly-L-lysine

The Mg-adenosinetriphosphatase (ATPase) in the thyroidal NaI-treated microsome fraction was activated by treatment with basic polyamino acids or trypsin, but not with acidic polyamino acids and basic proteins such as lysozyme and ribonuclease. The enzyme kinetics showed that the activation of trypsin or poly-L-lysine was due to an increase in the maximal velocity of the hydrolyzing reaction without a change in the affinity of the enzyme for its substrate. A break at about 25 degrees C was observed in the Arrhenius plots of Mg-ATPase in the trypsin- or poly-L-lysine treated preparations, but there was no break in the control preparation. These results suggest that the activating effect of trypsin or poly-L-lysine on Mg-ATPase activity in the thyroidal NaI-treated microsome fraction is related to the lipid environment surrounding the enzyme molecule in the thyroid cell membrane.