Complementarity of ATP-induced and light-induced absorbance changes around 515 nm

A comparative study of the light-induced and the ATP-induced changes of P-515 absorbance gave the following results: (1) Following light activation of the latent ATP-hydrolase, ATP can induce a ΔA(515) of about the same size as that observed either in continuous light or by a saturating light flash. The ATP-induced ΔA(515) is stable in the dark as long as ATP is hydrolysed. (2) Any preceding ATP-induced ΔA(515) reduces the size of a consequent light-induced ΔA(515), and vice versa. The total P-515 absorbance change which can be induced by ATP and light is constant; there is strict complementarity of ATP- and light-induced ΔA(515). (3) The suppression of the flash-induced ΔA(515) by a preceding ATP-induced ΔA(515) is accompanied by an about 15-fold acceleration of the overall dark-decay rate, which is not further accelerated by addition of 0.2 μM valinomycin. (4) Adopting the kinetic model of Schapendonk (Doctoral Thesis, Wageningen, 1980) it is concluded that the apparent acceleration of the overall dark-decay rate results from a specific elimination of the slowly decaying ‘Reaction II’ component. ATP hydrolysis is suggested to produce and to maintain the Reaction II-type electrochromic pigment shift in the dark. (5) The data offer an alternative explanation to the prevailing notion that increased proton conductance via the activated ATPase is the main cause for the apparent acceleration of the overall decay rate of the flash-induced ΔA(515) following preillumination or under ‘phosphorylating conditions’. (6) On the basis of the presented data it is argued that the total number of available sites which can produce a Reaction II-type electrochromic pigment shift is strictly limited. Consequently, the notion of a localized ATP- or light-induced field is favored. The properties of this localized field would suggest a close link to energy-dependent changes at the coupling factor complex and to the electrogenic reactions coupled with cyclic photophosphorylation.