Identification of Photosynthesis Characteristics and Chlorophyll Metabolism in Leaves of Citrus Cultivar (Harumi) with Varying Degrees of Chlorosis.

In autumn and spring, citrus leaves with a Ponkan (Citrus reticulata Blanco cv. Ponkan) genetic background (Harumi, Daya, etc.) are prone to abnormal physiological chlorosis. The effects of different degrees of chlorosis (normal, mild, moderate and severe) on photosynthesis and the chlorophyll metabolism of leaves of Citrus cultivar (Harumi) were studied via field experiment. Compared with severe chlorotic leaves, the results showed that chlorosis could break leaf metabolism balance, including reduced chlorophyll content, photosynthetic parameters, antioxidant enzyme activity and enzyme activity related to chlorophyll synthesis, increased catalase and decreased enzyme activity. In addition, the content of chlorophyll synthesis precursors showed an overall downward trend expected for uroporphyrinogen III. Furthermore, the relative expression of genes for chlorophyll synthesis (HEMA1, HEME2, HEMG1 and CHLH) was down-regulated to some extent and chlorophyll degradation (CAO, CLH, PPH, PAO and SGR) showed the opposite trend with increased chlorosis. Changes in degradation were more significant. In general, the chlorosis of Harumi leaves might be related to the blocked transformation of uroporphyrinogen III (Urogen III) to coproporphyrinogen III (Coprogen III), the weakening of antioxidant enzyme system activity, the weakening of chlorophyll synthesis and the enhancement in degradation.

Human uroporphyrinogen III synthase: NMR-based mapping of the active site.

Uroporphyrinogen III synthase (URO-synthase) catalyzes the cyclization and D-ring isomerization of hydroxymethylbilane (HMB) to uroporphyrinogen (URO'gen) III, the cyclic tetrapyrrole and physiologic precursor of heme, chlorophyl, and corrin. The deficient activity of human URO-synthase results in the autosomal recessive cutaneous disorder, congenital erythropoietic porphyria. Mapping of the structural determinants that specify catalysis and, potentially, protein-protein interactions is lacking. To map the active site and assess the enzyme's possible interaction in a complex with hydroxymethylbilane-synthase (HMB-synthase) and/or uroporphyrinogen-decarboxylase (URO-decarboxylase) by NMR, an efficient expression and purification procedure was developed for these cytosolic enzymes of heme biosynthesis that enabled preparation of special isotopically-labeled protein samples for NMR characterization. Using an 800 MHz instrument, assignment of the URO-synthase backbone (13)C(alpha) (100%), (1)H(alpha) (99.6%), and nonproline (1)H(N) and (15)N resonances (94%) was achieved as well as 85% of the side-chain (13)C and (1)H resonances. NMR analyses of URO-synthase titrated with competitive inhibitors N(D)-methyl-1-formylbilane (NMF-bilane) or URO'gen III, revealed resonance perturbations of specific residues lining the cleft between the two major domains of URO synthase that mapped the enzyme's active site. In silico docking of the URO-synthase crystal structure with NMF-bilane and URO'gen III was consistent with the perturbation results and provided a 3D model of the enzyme-inhibitor complex. The absence of chemical shift changes in the (15)N spectrum of URO-synthase mixed with the homogeneous HMB-synthase holoenzyme or URO-decarboxylase precluded occurrence of a stable cytosolic enzyme complex.

19-Bromo-1-hydroxymethylbilane, a novel inhibitor of uro'gen III synthase.

A novel hydroxymethylbilane analog, 19-Br-HMB (11), has been synthesized. Its activity with the enzyme Uro'gen III synthase shows competitive inhibition.

Allosteric inhibition of human lymphoblast and purified porphobilinogen deaminase by protoporphyrinogen and coproporphyrinogen. A possible mechanism for the acute attack of variegate porphyria.

Variegate porphyria (VP) is characterized by photocutaneous lesions and acute neuropsychiatric attacks. Decreased protoporphyrinogen oxidase activity results in accumulation of protoporphyrin (ogen) IX and coproporphyrin (ogen) III. During acute attacks delta-aminolevulinic acid and porphobilinogen also increase, suggesting that porphobilinogen deaminase (PBG-D) may be rate limiting. We have examined the effects of porphyrinogens accumulating in VP on PBG-D activity in Epstein-Barr virus-transformed lymphoblast sonicates from 12 VP and 12 control subjects. Protoporphyrinogen oxidase activity was decreased and protoporphyrin increased in VP lymphoblasts. PBG-D in control lymphoblasts obeyed Michaelis-Menten kinetics (Vmax 28.7 +/- 1.8 pmol/mg per h, Hill coefficient 0.83 +/- 0.07). VP sonicates yielded sigmoidal substrate-velocity curves that did not obey Michaelis-Menten kinetics. Vmax was decreased (21.2 +/- 2.0 pmol/mg per h) and the Hill coefficient was 1.78 +/- 0.17. Addition of protoporphyrinogen IX and coproporphyrinogen III to control sonicates yielded sigmoidal PBG-D substrate-velocity curves and decreased PBG-D Vmax. Addition of porphyrins or uroporphyrinogen III did not affect PBG-D activity. Removal of endogenous porphyrin (ogens) from VP sonicates restored normal PBG-D kinetics. Purified human erythrocyte PBG-D obeyed Michaelis-Menten kinetics (Vmax 249 +/- 36 nmol/mg per h, Km 8.9 +/- 1.5 microM, Hill coefficient 0.93 +/- 0.14). Addition of protoporphyrinogen yielded a sigmoidal curve with decreased Vmax. The Hill coefficient approached 4. These findings provide a rational explanation for the increased delta-aminolevulinic acid and porphobilinogen during acute attacks of VP.

Studies on the active centre of rat liver porphyrinogen carboxylase in vivo effect of hexachlorobenzene.

1. Porphyrinogen carboxylase from the liver of normal and hexachlorobenzene porphyric rats was subjected to chemical modification using photo-oxidation with methylene blue, diethylpyrocarbonate, butane-2,3-dione, and phenylglyoxal. 2. All of these chemicals inactivated the enzyme from both sources. 3. Reversion of the diethylpyrocarbonate reaction with hydroxylamine as well as protection of the enzymes with uroporphyrinogen III indicated that histidine is involved at least in the first decarboxylation active site of the porphyrinogen carboxylyase, and perhaps in one or more sites where the removal of the other carboxyl groups take place. 4. Arginine seems not to be at the active site of the enzyme but at its environment since two diketones alter the enzyme activity, however the substrate did not protect the enzyme from the butane-2,3-dione modification. 5. Comparative studies between the enzyme from normal and porphyric animals suggest that the low enzyme activity from intoxicated animals could be due to alterations of its active centre environment produced by hexachlorobenzene treatment. This treatment seems to partially protect the active site of the porphyrinogen carboxylase from the modification reactions.

Stimulation of 5-aminolevulinic acid metabolism by uroporphyrinogen I in rat liver homogenate.

The effect of excess uroporphyrinogens I and III, coproporphyrinogen III, and the corresponding porphyrins, on the rate of 4-14C-5-aminolevulinic acid (ALA) metabolism was studied. Experiments were performed with mitochondria-free rat liver homogenates prepared from normal rats. The consumption of labelled 5-aminolevulinic acid was followed by measuring its level in aliquots removed at intervals. The pattern of porphyrinogen synthesis was examined by high pressure liquid chromatography. Only uroporphyrinogen I had an effect; it increased the rate of conversion of ALA and porphobilinogen (PBG) to porphyrinogens. Chromatographic analysis revealed increased synthesis of uroporphyrinogen and heptacarboxylic porphyrinogen. It is believed that this mechanism might explain the lack of ALA and PBG accumulation in erythropoietic porphyria and porphyria cutanea tarda, and the absence of acute porphyria attacks in these conditions.

Decreased liver delta-aminolaevulinate dehydratase activity in porphyria cutanea tarda and in alcoholism.

delta-Aminolaevulinate dehydratase activity was measured in liver biopsy specimens from 13 patients with porphyria cutanea tarda, 13 alcoholics and 10 control subjects. The mean enzyme activities in liver of porphyria cutanea tarda and alcoholics were 34% (P less than 0.001) and 49% (P less than 0.001) respectively of the control level. Heat treatment of liver supernatant caused slight inhibition of the enzyme activity in porphyria cutanea tarda and in controls. Addition of Zn2+ to liver supernatants slightly increased the enzyme activity in both porphyria cutanea tarda and controls. The addition of liver supernatant from the porphyria cutanea tarda to purified bovine delta-aminolaevulinate dehydratase did not result in suppression of the enzyme activity. The apparent Km value of delta-aminolaevulinate dehydratase was 4.3 X 10(-4) mol/l for porphyria cutanea tarda livers and 3.3 X 10(-4) mol/l for control livers. The difference between the two was not significant.

Some kinetic properties of human red cell uroporphyrinogen decarboxylase.

Several kinetic properties of uroporphyrinogen decarboxylase (uroporphyrinogen-III carboxy-lyase, EC 4.1.1.37) from human hemoglobin-free hemolysates were studied, using substrates of both isomeric series I and III (uroporphyrinogen, hepta and pentacarboxyl porphyrinogens). Enzyme affinity for series II isomers was always found to be higher than for corresponding series I isomers. Mixed substrate experiments using porphyrinogen (both labelled with 14C and unlabelled) showed: (a) a reciprocal inhibition of decarboxylation of series III porphyrinogens by series I porphyrinogens with the same number of carboxylic groups; (b) no inhibition of hepta- and pentacarboxylic series III porphyrinogens decarboxylation by uroporphyrinogen III. It is demonstrated that porphyrinogens of both isomeric series with the same number of carboxylic groups are decarboxylated at the same active center; in contrast, the sequential decarboxylation of uroporphyrinogen III to coproporphyrinogen III occurs at four different active centers. Relationship between the kinetic properties of uroporphyrinogen decarboxylase and biological data of porphyria cutanea are discussed.