Acta Biochimica et Biophysica 4. (1969)

1969 / 4. szám - Gabriella Meszes-L. Erdei: The Effect of pH on the ATPase Activity of the Unicellular Alga Scenedesmus obtusiusculus CHOD

362 G. Meszes, L. Erdei: Effect of pH on the ATPase Activity As shown in Fig. 6, the ATPase activities of the intact cells and chloroplasts are quantitatively almost identical. This coincidence may suggest that the enzyme activity of the intact cells at this pH-value originates from the enzyme located in the chloroplast fraction. Since at this pH the (Mg2+—K+—Na+)-activated ATP­ase of the intact cells was inhibited by ouabain, ouabain would inhibit also the chloroplast enzyme at pH 7.2 if the enzyme was involved in both cases. Therefore we examined the effect of ouabain on the ATPase activity of the chloroplasts. However, since ouabain had no effect on the ATPase activity of the chloroplast fraction at various pH values there are presumably two enzymes both functioning optimally at pH 7.2: an ouabain-sensitive enzyme outside the chloroplasts and an ouabain-insensitive one located in the chloroplasts. The verification of this assump­tion requires further experiments. Fig. 6 shows also the enzyme activity of chloroplasts without added ions. There is practically no enzyme activity at any pH value. This can be due to the fact that the ionic concentration required for the activity of ATPase has been lost in the course of washing during the isolation of chloroplasts. The same result was obtained when tris-maleic acid-NaOH buffer was used. On the addition of ions ATPase activity could be detected only at pH 7.2. Ouabain had no inhibitory effect. Discussion According to literary data ATPase enzymes in plants have widely different pH optima. Thus e.g. Dodds and Ellis (1966) haveexamined the cation-stimulated ATPase activity in the cell-wall fraction of carrot and pea roots in the presence of MgCl2, KC1, and ATP in imidazole-acetate buffer. The optimum was observed at pH 4.5 (Dodds, Ellis, 1966). A Mg2+-dependent enzyme, activated by Na+ and inhibited by ouabain, i.e. similar to the transport ATPase of erythrocytes, has been found in the root tips of pine-tree. However, the pH optimum of this enzyme was different from that of animal ATPases: the optimum was at pH 5.5 instead of 7.2 (McClurkin, McClurkin, 1967). The pH optimum of the Mg2 + - dependent ATPase of the chloroplast of Euglena gracilis, one of the active ATPase from this source, was foundto be 5.5 (Carell, Kahn, 1967). On the alkaline side the pH optima of the ATPases of chloroplasts fall between pH 7.4 and 8.4 (Bennun, Avron, 1964; Hoch, Martin, 1963; McCarthy, Racker, 1968). The Mg2+-activated ATPase of Escherichia coli had its pH optimum at 8.7 in the absence of KC1. In the presence of KC1 the specific activity increased and the optimum shifted to pH 9.1 (Hafkenscheid, Bonting, 1969). Günther and Dorn (1966) studied the pH-de­­pendence of ATPase activity in intact E. coli cells and observed the optimum at pH 8.5. In our present experiments we compared the (Mg2+—K+—Na+)-activated ATPase activities of Scenedesmus obtusiusculus in intact alga cells and in the chlo­roplasts at various pH values. Our results have shown that the ATPase activity of intact cells was strongly dependent on the pH. The highest activities were meas­ured at pH 5.6, 6.4 and 7.2. Ouabain exhibited an inhibitory effect only at pH 7.2. Consequently it seems reasonable to assume that there are more than one ATPase Acta Biochimica et Biophysica Academiae Scientiarum Hungaricae 4, 1969

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