Pigment-protein complexes derived from Rhodospirillum rubrum chromatophores by enzymatic digestion

Abstract

Rhodospirillum rubrum chromatophores are acted upon by pancreatin (a crude mixture of pancreatic digestive enzymes) or α-chymotrypsin in the presence of 0.5% Triton X-100 to produce three clearly defined pigment-protein complexes which are separable by sucrose density-gradient centrifugation. The first to appear temporally is a brown band which contains carotenoids in an unusual combination, since the complex does not show the absorption peaks in the 400 to 500 nm range normally shown by carotenoids in the chromatophore. The complex does show a large absorption band with a maximum at 368 nm. The addition of acetone and methanol to this complex allows the usual absorption properties of the carotenoids to appear. Two bacterio-chlorophyll (Bchl)-protein complexes are formed. A blue Bchl-protein complex has absorption maxima at 922, 835, 580 and 372 nm. The 922 nm band is partially bleached by addition of ferricyanide, indicating there are two Bchl types in the complex. Addition of ascorbate largely restores the 922 nm peak. The band at 372 nm is due to some carotenoids which are present. A green Bchl-protein complex has absorption maxima at 780, 587 and 360 nm. The green complex (but not the blue) is active in simple photochemical electron transfer reactions such as reduced phenazine metho-sulfate oxidation in the presence of ubiquinone and the photoreduction of 5,5′-dithiobis-(2-nitrobenzoic acid) in the presence of ascorbate and 2,6-dichlorophenolindo-phenol. The green complex is more labile than the blue complex once they are formed. The Bchl-protein complexes could represent modified Bchl-protein complexes which exist in situ or could be formed from Bchl and proteins which are liberated by the digestive enzymes. Treatment with α-chymotrypsin or pancreatin produces similar complexes from R. rubrum chromatophores. Treatment of a blue-green mutant of R. rubrum which lacks carotenoids yields the blue and the green bands, but shows no brown band following centrifugation. Treatment of Chromatium chromatophores with either pancreatin or α-chymotrypsin in the presence of Triton X-100 results in complete digestion of the chromatophore with no discernible pigment complexes being formed.