多粘菌素B对光合磷酸化的促进作用

Stimulatory Effects of Polymixin B on Photophosphorylation

多粘菌素B在低浓度的磷酸盐存在下,抑制光合磷酸化和电子传递,但在较高浓度的磷酸盐存在下,却能促进光合磷酸化和提高P/O值。多粘菌素B在促进光合磷酸化时与无机磷酸之间存在着类似竞争的关系,并显示出能增加叶绿体的毫秒延迟发光和用中性红表示的类囊体膜内外的pH变化,多粘菌素B亦能促进光下的ATP_P_j交换。根据上述实验结果;我们推测多粘菌素B在类囊体膜上可能有两个作用部位,一个在类囊体膜上,另一个在偶联因子(CF_1)的β亚单位上。

The effects of polymyxin B on photophosphorylation and the ATP-Pi exchange reaction were studied. 1. Polymyxin B decreased the rate of photophosphorylation and the activity of electron transport in low concentration of phosphate (Table 1), but increased the rate of photophosphorylation and P/O ratio in high concentration of phosphate (Fig. 1, Table 2). In this reaction there was an almost competitive relationship between polymyxin B and phosphate (Fig. 2). 2. Polymyxin B increased the activities of both cyclic and non-cyclic photophosphorylation in the presence of high concentration of phosphate under low or high light intensity (Table 4, 5). These results show that neither the pathway nor the rate of electron transport affects the stimulation of photophosphorylation by polymyxin B. 3. While polymyxin B stimulated photophosphorylation, it reduced the high energy state formation measured by the method of two stage photophosphorylation (not shown). On the other hand, polymyxin B raised the msdelayed light emission of chloroplasts (Fig. 3) and promoted the internal pH changes of thylakoids measured with neutral red, but it brought down the 515 nm absorbance changes of chloroplasts (Fig. 4). 4. We examined the effects of polymyxin B on the ATP-Pi exchange reaction which is thought to consist of complete cycles of ATP synthesis and hydrolysis. Stimulation did occur. From the above results it is suggested that there be two types of action of polymyxin B on thylakoid. One is to increase the permeability of thylakoid membrane, and thereby to reduce the high energy state formation and accelerate the decay of membrane electron potential (Fig. 4B). The other is to act on the β subunit of CF_1. In the presence of high concentration of phosphate it interacts with β subunit of CF_1, inducing the conformational change of CF_1, and thus reducing the loss of ene gy in non-productive way through CF_1. The energy saved might be used to enhance ATP formation and to raise the P/O ratio.