Stimulatory Effects of Polymixin B on Photophosphorylation
摘要
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 A