NaCl(2 mol/L)处理PSⅡ颗粒的Ca^(2+)重结合

Studies on the Characteristics of Ca^(2+) Reconstitution in NaCl (2 mol/L)-Washed Photosystem Ⅱ Particles

回加Ca^(2+)对 NaCl(2 mol/L)处理菠菜PSⅡ颗粒放氧活性的作用表现出有两种K_m值分别为 0.021和 0.545 mmol/L的高亲和与低亲和的Ca^(2+)重结合过程。高浓度NaCl和低 pH(3.0)处理去Ca的 PSⅡ颗粒的光抑制放氧活性半衰期(t_(1/2))明显减小。重结合Ca^(2+后,虽然大部办丧失的放氧活性可恢复,但PSⅡ颗粒放氧活性对光抑制的敏感性却并不随之恢复,t_(1/2)无明显改变。显然,重组Ca^(2+)的结合状态和作用与PSⅡ颗粒原有的结合Ca不完全相同。

In recent years, it has been dis-covered that NaCl (2 mol/L)-washingof PS Ⅱ particles resulted in strongsuppression of oxygen evolution whichcould be restored by the addition ofCa^(2+). However, the characteristics ofCa^(2+) reconstitution and the mecha-nisms of Ca^(2+)-dependent reactivationare still unknown. In the presentwork, by analysing the dynamical fea-tures of Ca^(2+)-dependent reactivation ofNaCl-washed PS Ⅱ particles and com-paring the sensitivity of oxygen evolu-tion to photoinhibition before and afterthe addition of Ca^(2+), we discussed thequestion whether reconstituted Ca^(2+)played the role of Ca^(2+) bound original-ly which could be extracted by NaCl-washing. Washing PS Ⅱ particls with 2mol/L Of NaCl at 0℃ for 30 min inthe dark strongly depressed oxygenevolution concomitant with extensiveloss of two water-soluble polypeptides(17 kD and 23 kD) (Fig. 1). The ad-dition of Ca^(2+) to NaCl-washed PS Ⅱparticles markedly restored oxygenevolution (Table 1, Fig. 2). Dynam-ic analysis of this Ca^(2+) effect on oxy-gen evolution revealed both a high-(K_m= 0. 021 mmol/L) and a low-(K_m= 0. 545 mmol/L) affinity Ca^(2+)sites in the absence of Na^+ (Fig. 2).However, it is believed that NaCl-washing extracts only one Ca from areaction center of spinach PS Ⅱ parti- cles in which two Ca were found(Yocum 1991), therefore, one ormore binding sites of reconstitutedCa^(2+) in NaCl-washed PS Ⅱ particlesmust be different from the original Casites. On the other hand, strong illu-mination of PS Ⅱ particles led to anexponential decay of oxygen evolutionrate which indicated a single damagecomponent (Fig. 3). Judging fromthe half-times, NaCl-washed PS Ⅱparticles Showed stronger susceptibilityof oxygen evolution to photoinhibitionthan untreated ones. In contrast to therestoration of oxygen evolution, thehalf-times of NaCl-washed PS Ⅱ parti-cles under photoinhibition did notchange significantly after the additionof Ca^(2+). That is, Ca^(2+) reconstitutiondid not reverse the increased suscepti-bility of NaCl-washed PS 1 particles.Obviously, the binding state of recon-stituted Ca^(2+) in NaCl-washed PS Ⅱparticles is different from that of theoriginal Ca functioning in oxygen evo-lution, and the restoration of oxygenevolution is not due to the rebinding ofCa^(2+) to the original functional sitefrom which Ca has been removed byNaCl-washing.