[Changes in the light-absorption of a carotenoid in an enzyme (de-epoxidase)-substrate(Violaxanthin)-complex]

The enzyme violaxanthin de-epoxidase catalysing the transformation of the xanthophyll violaxanthin to zeaxanthin has been isolated from spinach chloroplasts.Special properties of the enzyme make it possible for the carotenoid to be bound without initiation of any catalytic reaction; the isolation of an enzyme-substratecomplex is thereby greatly facilitated. After addition of cofactors to this complex the transformation of violaxanthin to zeaxanthin takes place.In this complex the light-absorption of violaxanthin is changed drastically: the normal three-peak absorption curve in the blue region of the spectrum is strongly decreased but in the uv-region around 380 nm a new absorption maximum appears.Recently a similar spectrum has been determined in vivo in the phototropic sensitive region of the sporangiophores of Phycomyces (Wolken, 1969) with the aid of microspectrophotometry.From these results it is concluded that part of the carotenoids occurring in plants is present in a protein-bound form and that these pigments show a considerably changed light absorption in comparison with the isolated pigment. The simultaneous occurrence of differently bound carotenoids may lead to the formation of 4-peak absorption curves (similar to those of flavines) with 3 maxima in the blue region and 1 maximum in the UV around 370-380 nm. These 4-peak curves are characteristic for many action spectra.It is emphasized that the strong absorption changes of carotenoids occurring during the binding of these pigments to proteins should be considered in analyzing difference spectra.