Influence of light and calcium on guanosine 5'-triphosphate in isolated frog rod outer segments.
Influence of light and calcium on guanosine 5'-triphosphate in isolated frog rod outer segments.
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DOI：
10.1085/jgp.74.6.649
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发表时间：
1979-12
影响因子：
3.8
通讯作者：
Bownds, M D
中科院分区：
文献类型：
--
作者：
Biernbaum, M S;Bownds, M D
青蛙视紫红质外节在脱离视网膜3分钟后，每摩尔视紫红质约含有0.25摩尔的GTP和0.25摩尔的三磷酸腺苷。UTP和CTP分别处于低10倍和100倍的水平。在接下来的4分钟内，GTP和ATP的浓度同时下降，达到相对稳定的水平，即每摩尔视紫红质0.1mol。光照降低了内源GTP的浓度，但不降低ATP的浓度。这种光诱导的GTP下降可以高达70%，半衰期为7 S。在中等强度的延长光照下，GTP下降到稳定的中间水平，但在短暂的光照后，GTP部分恢复到暗适应水平。在每秒5×10(2)到5×10(6)个视紫红质分子/外段之间漂白的水平下，减少的幅度以连续光强度的对数的线性函数增加。这超出了光照导致孤立外节的循环GMP含量和渗透率下降的强度范围(Woodruff和Bownds)。1979年。J.Gen.Physiol.73：629-653)。因此，超过4个对数单位的光强度时，灵敏度控制机构起作用以使扩展照明在刺激GTP降低方面不那么有效。暗适应外节中的GTP水平对悬浮介质中钙浓度的变化很敏感。如果外界钙浓度降低到10(-8)M，GTP浓度将降低到饱和光照引起的相同水平，剩余的GTP不再对光敏感。将钙浓度降低到10(-6)-10(-8)M之间的中间水平会将GTP降低到稳定的中间水平，剩余的GTP可以在光下减少。恢复毫米级钙促进GTP的合成，但不促进ATP的合成，GTP对光照的不稳定性再次被观察到。通过添加二价阳离子载体A23187，这些钙诱导的GTP变化被减弱。降低或提高镁水平不影响GTP浓度。这些数据增加了光激活由GTP水解酶驱动的钙转运机制，或其他未知功能的钙敏感的GTP酶活性的可能性。已知的涉及环核苷酸转换和视紫红质磷酸化的光依赖反应似乎只占光诱导的GTP下降的一小部分。
Frog rod outer segments contain approximately 0.25 mol of GTP and 0.25 mol of ATP per mol of rhodopsin 3 min after their isolation from the retina. UTP and CTP are present at 10-fold and 100-fold lower levels, respectively. Concentrations of GTP and ATP decline in parallel over the next 4 min to reach relatively stable levels of 0.1 mol per mol of rhodopsin. Illumination reduces the concentration of endogenous GTP but not ATP. This light-induced decrease in GTP can be as large as 70% and has a half-time of 7 s. GTP is reduced to steady intermediate levels during extended illumination of intermediate intensity, but partially returns to its dark-adapted level after brief illumination. The magnitude of the decrease increases as a linear function of the logarithm of continuous light intensity at levels which bleach between 5 X 10(2) and 5 X 10(6) rhodopsin molecules/outer segment per second. This exceeds the range of intensities over which illumination causes decreases in the cyclic GMP content and permeability of isolated outer segments (Woodruff and Bownds. 1979. J. Gen. Physiol. 73:629-653). Thus, over 4 log units of light intensity, a sensitivity control mechanism functions to make extended illumination less effective in stimulating a GTP decrease. GTP levels in dark-adapted outer segments are sensitive to changes in calcium concentration in the suspending medium. If the external calcium concentration is reduced to 10(-8) M, GTP concentration is lowered to the same level caused by saturating illumination, and the GTP remaining is no longer light-sensitive. Lowering calcium concentration to intermediate levels between 10(-6) and 10(-8) M reduces GTP to stable intermediate levels, and the GTP remaining can be reduced by light. Restoration of millimolar calcium drives synthesis of GTP, but not of ATP, and GTP lability towards illumination is again observed. These calcium-induced changes in GTP are diminished by the addition of the divalent cation ionophore A23187. Lowering or raising magnesium levels does not influence the GTP concentration. These data raise the possibility that light activates either a calcium transport mechanism driven by the hydrolysis of GTP, or some other calcium-sensitive GTPase activity of unknown function. Known light-dependent reactions involving cyclic nucleotide transformations and rhodopsin phosphorylation appear to account for only a small fraction of the light-induced GTP decrease.