Pigment Pool Research Articles

Chlorophyll Turnover in Skeletonema costatum, a Marine Plankton Diatom

[(3)H]- and delta-[(14)C]Aminolevulinic acids were incorporated into the chlorophylls of Skeletonema costatum, a marine plankton diatom. In the stationary phase of growth, the tetrapyrrole-based pigments reached steady-state labeling after 10 hours. Under conditions of exponential cell division and chlorophyll accumulation, (3)H was rapidly lost from the labeled chlorophylls and was replaced with (14)C derived from delta-[4-(14)C]aminolevulinic acid. The kinetics of isotope dilution suggests recycling of tetrapyrrole precursors and/or two pigment pools, containing both chlorophyll a and chlorophyllide c, one which turns over rapidly (10 hours) and another which turns over more slowly (100 hours). Calculation of turnover times varied from 3 to 10 hours for chlorophyll a and from 7 to 26 hours for chlorophyllide c. The data suggest the dynamics of chlorophyll metabolism in S. costatum and explain the diatom's ability to undergo light-shade adaptation within a generation time.

Evidence for a light-induced blue band shift of part of the p515 pigment pool in intact chloroplasts

The difference spectrum of light-induced absorbance changes in the 460-540 nm wavelength region, measured in algae and chloroplasts, is characterized by a maximum around 518 nm, a minimum at about 480 nm and an inflection in the 490-500 nm wavelength region [l-4]. Evidence has been given that this spectrum is the result of an electrochromic response of chlorophyll b and carotenoid pigments to the electric field generated by the primary light-induced charge separation in the membrane [5,6]. It has been suggested that, at least in bacterial chromatopho:ts, the pool of carotenoids that shows electrochromism may not be completely homogeneous and consists of molecules with slightly different peak wavelengths and bandshifts depending on the strength and orientation of local electric fields [7]. In this paper we discuss an effect of cations on the light-induced difference spectrum of P5 15 in chloroplasts. It is shown that in intact chloroplasts as well as in broken chloroplasts in the presence of cations part of the pigments in the pool respond in a way that upon flash illumination their absorption band is shifted towards a shorter wavelength. This blue wavelength shift, which gives rise to the inflection around 500 nm in the overall difference spectrum, is interpreted in terms of a cation-dependent change in the orientation of the pigment complex with respect to an induced photoelectric field, or of a cation-induced migration of the primary acceptor of PS II into the hydrophobic region of the membrane. This would suggest that a small part of the pigments might become exposed in the light to an oppositely oriented

Hepatic accumulation and intracellular binding of conjugated bilirubin.

After the intravenous injection of unconjugated [(3)H]bilirubin into normal Sprague-Dawley and Wistar R rats, radiolabeled bile pigments rapidly accumulated in the liver. By 1.5 min after injection, an average of 36% of the injected isotope was present in liver homogenates. Between 3 and 15 min, 37-64% of the total intrahepatic radiolabeled bilirubin was conjugated, as demonstrated by extraction of label into the polar phase of a solvent partition system. This indicates both rapid conjugation, and accumulation of conjugated bilirubin within the liver cell. Fluorometric determination of the dissociation constants of purified bilirubin and its mono- and diglucuronides for homogeneous preparations of two human and four rat glutathione S-transferases, including ligandin, revealed avid binding of all three bile pigments to this class of proteins. Hence, the observation that the intrahepatic bile pigment pool contains substantial amounts of conjugated bilirubin can be attributed to the high binding affinities observed. Thin-layer chromatographic analysis of the (3)H-pigments produced by p-iodoaniline diazotization of homogenates and cytosol demonstrated that the intrahepatic pool of conjugated bilirubin was almost exclusively monoglucuronide. Examination of radiolabeled bilirubin conjugates excreted in bile during the first 20 min after injection of [(3)H]bilirubin showed no preferential excretion of diglucuronide. These studies indicate that (a) both bilirubin and its monoglucuronide accumulate within the liver cell as ligands with the glutathione S-transferase; and (b) bilirubin diglucuronide does not significantly accumulate within the general intrahepatocellular pool of protein-bound bile pigments. The latter observation is compatible with the formation and excretion of bilirubin diglucuronide directly from the canalicular pool of the liver cell.