Function of plastoquinones B and C as electron acceptors in Photosystem IIand fatty acid analysis of plastoquinone B

We have found that in petroleum-ether extracted tobacco thylakoids, plastoq uinone A (PQ-A) and plastoquinone C (PQ-C) had similar efficiency in restor ation of oxygen-evolving activity, while plastoquinone B (PQ-B), which is a fatty acid ester of PQ-C, was about 50% less effective. This indicates tha t apart from PQ-A, PQ-C and to a smaller extent PQ-B may function as electr on accepters of Photosystem II (PS II). The DCMU inhibition curves for PQ-C and PQ-B were biphasic and an initial slow decline was followed by a sharp decrease in oxygen evolution yield with a 50% inhibition (I-50) at 0.25 mu M DCMU. In the case of PQ-A (I-50 = 0.20 mu M DCMU), the activity decrease d gradually without the sharp transition. The corresponding inhibition curv e for unextracted thylakoids, where all the native prenylquinones are prese nt, shows an intermediate shape between PQ-A and PQ-C but with a higher I-5 0, equal to 0.32 mu M, suggesting that the contribution of PQ-C as an elect ron acceptor of Photosystem II might be significant in thylakoid membranes with natural prenyllipid composition. alpha-Tocopherol quinone showed no ac tivity in the restoration of oxygen evolution in extracted thylakoids, indi cating that it cannot accept electrons from PS II. The fatty acid compositi on of PQ-B isolated from maple leaves showed a high degree of saturated fat ty acids like myristic and palmitic acid, and its unique composition indica tes that it is a natural component of the thylakoid membrane.