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R B Frydman

Publications and source records attributed to R B Frydman.

At least 55 records · Page 3Linked to original sources
PubMed1981

Specificity of heme oxygenase: a study with synthetic hemins.

A large number of synthetic iron porphyrins were enzymatically oxidized by a microsomal heme oxygenase preparation from rat liver. They all had in common two vicinal propionic acid residues at C6 and C7. Iron porphyrins of type I were not substrates of the enzyme. Iron porphyrins that carried electron-withdrawing substituents (acyl residues) at C2 and C4 were substrates of heme oxygenase, although the product yields were reduced. Several iron porphyrins, such as hemin XIII (4) and hem III (5), were better substrates of heme oxygenase than the natural substrate hemin IX (1). The enzymatic oxidation was selective for the alpha-methine bridge, and the alpha-biliverdins obtained were reduced by Biliverdin reductase to the corresponding alpha-bilirubins. Preincubation of the enzymatic system with hemin IX (1) and hemin XIII (4) in the absence of NADPH resulted in an inhibition of their oxidation. The iron-free porphyrins which carried two vicinal propionic acid residues at C6 and C7 were also found to be inhibitors of the enzymatic system when preincubated with the latter. The presence of hematochemin IX (18) suppressed the enzymatic oxidation of hemin IX (1).

Animals↗
PubMed1981

Induction of porphobilinogen oxygenase and porphobilinogen deaminase in rat blood under conditions of erythropoietic stress.

Porphobilinogen is the substrate of two enzymes: porphobilinogen deaminase and porphobilinogen-oxygenase. The first one transforms it into the metabolic precursors of heme and the second diverts it from this metabolic pathway by oxidizing porphobilinogen to 5-oxopyrrolinones. Rat blood is devoid of porphobilinogen-oxygenase under normal conditions while it carries porphobilinogen-deaminase activity. When the rats were submitted to hypoxia (pO2 = 0.42 atm) for 18 days, the activity of porphobilinogen-oxygenase appeared at the tenth day of hypoxia and reached the maximum at the 14-16th day. It decreased to a half after 2 days (half-life of the enzyme) and disappeared after 4 days of return to normal oxygen pressure. Porphobilinogen-deaminase activity increased after the first day of hypoxia, reached a maximum at the 14-16th day and did not decrease to normal values until the 15th day after return to normal oxygen pressure. The activities of both porphobilinogen-oxygenase and porphobilinogen-deaminase were induced by administration of erythropoietin. When rats were made anaemic with phenylhydrazine, porphobilinogen-oxygenase activity also appeared in the blood cells. Although the reticulocyte concentration was higher when compared to that obtained under hypoxia, the activities of the oxygenase obtained under both conditions were comparable. Porphobilinogen-deaminase activity was always closely related to the reticulocyte content. The appearance of porphobilinogenase-oxygenase under the described erythropoietic conditions was due to a de novo induction of the enzyme, as shown by its inhibition with actinomycin D and cycloheximide. Porphobilinogen-oxygenase as well as porphobilinogen-deaminase were present in the rat bone marrow under normal conditions. Their activities increased in phenylhydrazine treated rats. The properties and kinetics of porphobilinogen-oxygenase from the rat blood and bone marrow were determined and found it differ in several aspects.

Ammonia-Lyases↗
PubMed1979

Porphobilinogen oxygenase from human erythrocytes.

Porphobilinogen oxygenase was isolated from red cells of healthy persons and of patients with disturbances in porphyrin metabolism. The hemolysates were purified by DEAE-cellulose chromatography and the oxygenase was eluted with either 3 mmol/l phosphate buffer (pH 7.4) or with 10 mmol/l Tris-HCl buffer (pH 7.6). The oxygenase can also be isolated by filtration of the hemolysate through Sephadex G-100. In healthy persons a mean value of 84.1 +/- 29.7 nmol of porphobilinogen consumed in 30 min per ml of blood was obtained. In patients with hepatic porphyrias (acute intermittent porphyria and porphyria cutanea tarda) the activity of porphobilinogen oxygenase was very low. In the case with acute intermittent porphyria the activity was increased when measured after storage at 4 degrees C but never reached normal values. In the cases of porphyria cutanea tarda, oxygenase activity increased after recovery and reached normal values. In patients with erythropoietic porphyria and in anemias the activity of porphobilinogen oxygenase gave high values. The erythrocyte enzyme was found to be heterogeneous when compared with the enzyme of other sources. It was only partially succinylated and was inactivated after storage for a few days at 4 degrees C. Some preparations showed the usual allosteric kinetics (n = 1.6--2.0), although Michaelian kinetics were also often observed. The enzyme was inhibited by alpha, alpha'-dipyridyl and EDTA as well as by several metals.

Animals↗
PubMed1979

Disappearance of porphobilinogen deaminase activity in leaves before the onset of senescence.

The activity of porphobilinogen deaminase was measured in young and senescent or mature leaves of pepper (Capsicum annuum), and poinsettia (Euphorbia pulcherrima). Whereas high activity was found in the crude extracts of the young leaves, almost no activity was found in the extracts of senescent or mature leaves. The decrease in deaminase activity was not due to the presence of an isolatable inhibitor. By purifying the crude enzyme extracts from leaves of different ages on DEAE-cellulose columns it was shown that the decrease in deaminase activity was due to a real decrease in the amount of enzyme. Fruiting also decreased porphobilinogen deaminase activity. Several kinetic constants of the C. annuum deaminase were determined.

Journal Article↗
PubMed1978

Biosynthesis of uroporphyrinogens. Interaction among 2-aminomethyltripyrranes and the enzymatic system.

Many hypotheses on uroporphyrinogen biosynthesis advanced the possibility that 2-aminomethyltripyrranes formed by porphobilinogen deaminase are further substrates or uroporphyrinogen III co-synthase in the presence of porphobilinogen. These proposals were put to test by employing synthetic 2-aminomethyltripyrranes formally derived from porphobilinogen. None of them was found to be by itself a substrate of deaminase or of co-synthase in the presence of porphobilinogen. The tripyrranes chemically formed uroporphyrinogens by dimerization reactions, and the latter had to be deducted in control runs during the enzymatic studies. Two of the tripyrranes examined, the 2-aminomethyltripyrrane 7 and the 2-aminomethyltripyrrane 8, were found to be incorporated into enzymatically formed uroporphyrinogen III in the presence of porphobilinogen and of the deaminase-co-synthase system. While the former gave only a slight incorporation, the latter was incorporated in about 16%. No incorporation of 8 into uroporphyrinogen I was detected. On the basis of these results, and of the previous results obtained with 2-aminomethyldipyrrylmethanes, an outline of the most likely pathway of uroporphyrinogen III biosynthesis from porphobilinogen is given.

Ammonia-Lyases↗
PubMed1977

Porphyrin biosynthesis from prophobilinogen by duck blood hemolysate.

The formation of porphyrins from porphobilinogen by a duck blood hemolysate was examined. The system was found to form mainly protoporphyrin IX and hemin, and accumulated lesser amounts of uroporphyrins, hepatacarboxylic porphyrin, and coproporphyrins. By storage at -20 degrees the accumulation of uroporphyrins and heptacarboxylic porphyrin was increased. Both porphyrins were mainly the type III isomers. By addition of dithiothreitol the porphyrin pattern reversed to the original one formed by the fresh hemolysate. Addition of a number of amines also inhibited the decarboxylating system without affecting the original isomer distribution among the porphyrins. Addition of Fe2+ (3mM) did not affect the porphyrin pattern or the isomer distribution. Addition of Pb2+ (2.5 mM) partially inhibited the decarboxylating system, whereas at higher concentrations (4 mM) it increased the decarboxylation rate of the heptacarboxylic porphyrin. The obtained results are discussed in relation to porphyrin accumulation in porphyria cutanea tarda and in acquired hepatic porphyrias.

Ammonium Chloride↗
PubMed1976

Biosynthesis of uroporphyrinogens from porphobilinogen: mechanism and the nature of the process.

The enzymic self-polymerization of prophobilinogen gives rise to the cyclic tetrapyrroles uroporphyrinogen III and uroporphyrinogen I. The former is the precursor of all the natural porphyrins and chlorins. The formation of uroporphyrinogen III is catalysed by a dual enzymic system, porphobilinogen deaminase and uroporphyrinogen III cosynthase. Deaminase polymerizes four porphobilinogen units on the enzymic surface, without liberation of free intermediates into the reaction medium, and forms uroporphyrinogen I. Cosynthase enters into association with the deaminase, and acts as a 'specifier protein' of the latter, changing the mode of porphobilinogen condensation on the enzymic surface. The association is independent of the presence of substrate. While deaminase catalyses the head-to-tail condensation of the porphobilinogen units, the association deaminase-cosynthase catalyses the head-to-head condensation of the same units. As a result different enzyme-bound dipyrrylmethanes are formed form the beginning of the process, and this can be demonstrated by using synthetic dipyrrylmethanes and tripyrranes.

Ammonia-Lyases↗