ロドプシンの光褪色

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A quantitative analysis of photochemical bleaching of cattle rhodopsin was performed with the same apparatus as described in a previous paper (Hara, 1958). Optical density-time curves of rhodopsin bleached by 90 sec illumination (about 3,000 lux) are given in Fig. 1. Time course of bleaching differs in dependence on pH; as the pH increases, the density loss occurs more profoundly. The bleaching of rhodopsin at different pH is also illustrated in Fig. 2, in which the process is expressed as logarithm of density loss against time. It is suggested that the bleaching can be regarded as consisting of two processes, a primary process and a secondary process, and that an intermediate (M) must exist in the course of the conversion of rhodopsin (R) to indicator yellow (I). Assuming that each process (primary and secondary) is a first order reaction with velocity constant k_1 and k_2, respectively, the concentration of each component R, M and I found in solution after t sec exposure can given by equation (4), (5) and (6). If each extinction coefficient (at 503 mμ) is denoted by ε_R, ε_M and ε_I, the optical density of the solution at any time in the course of bleaching must be indicated by equation (7). For the secondary process of bleaching (where t is enough large), this equation can be simplified as equation (8) or (9); the latter gives a straight line having the slope equal to velocity constant k_2. On the other hand, for the primary process, we can also obtain equation (11) or (12), subtracting equation (8) from equation (7). Equation (12) also gives a straight line concerned with velocity constant k_1 alone. According to the consideration described above, the later stage of bleaching in the alkaline sample (Fig. 2, B) first well equation (10) as a straight line having the slope k_2, because ε_I is practically equal to zero. For the initial stage, plotting of the experimental values also yields a straight line whose slope can be denoted as k_1 according to equation (12) (Fig. 2, b). In the case of neutral and acidic samples, equation (9) is proved as expressing the secondary process very well, when 0.15 ε_R and 0.31 ε_R are respectively substituted for the value of ε_I, and moreover, plotting for the primary process also gives a straight line which runs parallel with that of the alkaline sample. Thus the theoretical treatment has been justified, and both reaction velocity constants k_1 and k_2 have been found to be independent of pH of the solution. When rhodopsin is bleached by 3,000 lux at 25℃, the ratio of k_1 to k_2 is estimated as about 7. From the results shown in Fig. 3, the velocity constant k_2 is proved to be affected by temperature, but k_1 is constant for all the experiments with constant luminosity. After all, it will be very probable in our experiments that the apparent loss of density for 10-20 sec from the start of exposure is mainly ascribed to the reaction R → M, while the loss on and after 1 min to the reaction M → I. None of ions such as Na^・, K^・, Mg^<・・>, Ca^<・・>, Cl', SO"_4, CO_3", CN', H_2PO_4' and HPO_4" can give any remarkable effect on the primary process of bleaching of rhodopsin solution (cf. Fig. 4).
社団法人日本動物学会の論文
1960-04-15

ロドプシンの光褪色

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