Axonemes were isolated from sperm of Colobocentrotus by a procedure involving two extractions with 1% Triton X-100 and washing The isolated axonemes contained 7 x 1015 g protein per µm of their length. Treatment of the axonemes with 0 5 M KCl for 30 min extracted 50–70% of the flagellar ATPase protein, dynein, and removed preferentially the outer arms from the doublet tubules. Almost all of the dynein (85–95%) could be extracted from the axonemes by dialysis at low ionic strength. In both cases the extracted dynein sedimented through sucrose gradients at 12–14S, and no 30S form was observed The enzymic properties of dynein changed when it was extracted from the axonemes into solution. Solubilization had a particularly marked effect on the KCl- and pH-dependence of the ATPase activity. The pH-dependence of soluble dynein was fairly simple with a single peak extending from about pH 6 to pH 10. The pH-dependence of bound dynein was more complex. In 0.1 M KCl, the bound activity appeared to peak at about pH 9, and dropped off rapidly with decreasing pH, reaching almost zero at pH 7; an additional peak at pH 10 0 resulted from the breakdown of the axonemal structure and solubilization of dynein that occurred at about this pH. A similar curve was obtained in the absence of KCl, except for the presence of a further large peak at pH 8 Measurement of the kinetic parameters of soluble dynein showed that both Km and Vmax increased with increasing concentrations of KCl up to 0.5 M When bound dynein was assayed under conditions that would induce motility in reactivated sperm (0 15 M KCl with Mg++ activation), it did not obey Michaelis-Menten kinetics, although it did when assayed under other conditions. The complex enzyme-kinetic behavior of bound dynein, and the differences between its enzymic properties and those of soluble dynein, may result from its interactions with tubulin and other axonemal proteins

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