life ~ 3 minutes and G-actin-ADP in a half life ~ 0,3 minutes. On the other hand, the stability of F-actin-ADP is not noticeably affected (Section V ). is not affected at all by a treatment with the Teflon-homogenizer. Apparently the decrease of the KC1-concentration from 10-1 M to 5 x 10~4 M considerably diminishes the strength of the bond between the actin monomers without immediately destroying the F-actin arrangement. The immediate ADP-ATP-exchange after the mechanical destruction of the F-actin arrangement proves that this exchange in F-actin does not take place only because of steric hindrance. ADP is present in F-actin apparently between the individual monomers so that EDTA, ATP and enzymes affecting ATP cannot approach ADP. Consequently it is not necessary to assume that the extraordinary stability of F-actin-ADP is due to a special kind of bond between actin monomers and nucleotide phosphate (Section V ).8. In the appendix it is shown that G-actin-ADP does not polymerize 15' after preparation if the aceton dried muscle powder is prepared at pH 8 to 9 instead of pH ~ 7.In einer früheren Mitteilung ** wurde gezeigt ( U l b r e c h t und M ita rb b .1), daß fortgesetzte R eini gung von Actinpräparaten auf der Ultrazentrifuge, durch Magnesiumfällung nach B Ä r a n y 2 oder durch geeignete isoelektrische U m fällung zu einem kon
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