The Alkaline Adenosine Triphosphatase Activity of 30S Dynein after Modification of the SH Groups1

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Abstract
An apparent ‘triphasic’ alteration of 30S dynein ATPase activity was produced by treatment with various amounts of NEM when the modification and subsequent ATPase assay were carried out at pH 7.4 and pH 10–10.2, respectively. The Mg-ATPase activity was markedly inhibited by modification of the most reactive SH groups with 10 μMNEM, although the same treatment had no significant effect on the activity when assayed at neutral pH. Increasing the NEM concentration to 0.3 mM largely restored the enzyme activity, but a further increase in NEM concentration inhibited the enzyme activity again. This unusual response of 30S dynein ATPase at pH 10–10.2 was accounted for by the results of Arrhenius plots of the enzyme activity at pH 10.1; the enzyme protein modified with not more than 10 μm NEM was not stable under the assay conditions (pH 10-10.2 at 25°C), whereas modification with 0.3 mM NEM stabilized 30S dynein against the assay conditions. The possible significance of the 10 μM NEM-induced inhibition of the 30S dynein alkaline ATPase activity is discussed in connection with the participation of SH groups of 30S dynein in the enzyme activity.

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