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A H Gibbs

Publications and source records attributed to A H Gibbs.

At least 37 records · Page 2Linked to original sources
PubMed1982

Formation of cobalt protoporphyrin by chicken hepatocytes in culture. Relationship to decrease of 5-aminolaevulinate synthase caused by cobalt.

Cobalt protoporphyrin generated from 5-amino[4-14C]laevulinate by homogenates or primary cultures of chick embryo liver exposed to CoCl2 was found to be radioactivity unextractable by acid/acetone, when extra protein was added. The activity of ferrochelatase was required for formation of cobalt protoporphyrin since inhibition of ferrochelatase with 3,5-diethoxycarbonyl-1,4-dihydrocollidine (in the presence of cycloheximide) inhibited formation of cobalt protoporphyrin and resulted in accumulation of protoporphyrin. Cobalt protoporphyrin was detected spectrophotometrically in hepatocyte cultures exposed to the combination of 2-allyl-2-isopropylacetamide and CoCl2: (1) as the pyridine haemochrome of the protein pellet remaining after acid-acetone extraction of the cells, or (2) as the material extracted from the protein pellet with acetic acid-pyridine-chloroform. The amount of cobalt protoporphyrin increased with increasing CoCl2 concentration as cellular haem declined. The decrease in haem was about equal to the amount of cobalt protoporphyrin that accumulated. 2-Allyl-2-isopropylacetamide and polychlorinated biphenyls were both powerful inducers of 5-aminolaevulinate synthase. The former led to protoporphyrin accumulation, whereas with the latter, uroporphyrin accumulated, probably due to a concomitant decrease in activity of uroporphyrinogen decarboxylase. The decrease in activity of 5-aminolaevulinate synthase produced by administration of CoCl2 was greater after treatment with 2-allyl-2-isopropylacetamide than after treatment with allylisopropylacetamide and 3,4,3',4'-tetrachlorobiphenyl. We conclude: (a) that cobalt protoporphyrin is readily formed in cultured hepatocytes, and (b) that its formation accounts for the action of cobalt on 5-aminolaevulinate synthase.

5-Aminolevulinate Synthetase↗
PubMed1980

Inhibition of protohaem ferro-lyase by N-substituted porphyrins. Structural requirements for the inhibitory effect.

N-Methyl mesoporphyrin was a powerful inhibitor of protohaem ferro-lyase in vitro, whereas N-ethyl mesoporphyrin and N-methyl coproporphyrin were not and neither was the newly described green pigment produced by giving rats ethylene. This suggests that the size of the substituent at a pyrrole nitrogen and also the number of carboxylic acid side chains of the substituted porphyrin are important for the inhibitory effect. Evidence that N-methyl mesoporphyrin inhibited the enzyme, whereas the ethylene-derived pigment did not, was also obtained in vivo.

5-Aminolevulinate Synthetase↗
PubMed1980

Inhibition of protohaem ferro-lyase in experimental porphyria. Isolation and partial characterization of a modified porphyrin inhibitor.

1. A modified porphyrin with inhibitory activity towards protohaem ferro-lyase was purified from the livers of mice treated with 3,5-diethoxycarbonyl-1,4-dihydrocollidine, and partially characterized. 2. The inhibitor can be labelled by 5-amino[4-14C]-laevulinate, suggesting that it originates from pre-labelled liver haem. No radioactivity from 3,5-diethoxycarbonyl-1,4-dihydro[2,6-14C]collidine can be recovered bound to the purified abnormal porphyrin when the radioactive drug is used to induce its formation. 3. Similar modified porphyrins isolated from the livers of animals treated with 2-allyl-2-isopropylacetamide, secobarbitone or 1-ethynylcyclohexanol did not exhibit inhibitory activity toward protohaem ferro-lyase. 4. The inhibition of protohaem ferro-lyase was progressive, could be slowed down by cooling and partially prevented by preincubating the enzyme with the porphyrin substrate. Once established, inhibition could not be reversed by addition of the substrate 5. These results suggest that the modified porphyrin irreversibly inhibits protohaem ferro-lyase and may be used as a sensitive and selective reagent to titrate the number of active centres of the enzyme.

Animals↗
PubMed1980

Drug-induced conversion of liver haem into modified porphyrins. Evidence for two classes of products.

A porphyrin with inhibitory activity towards protohaem ferro-lyase (EC 4.99.1.1) was isolated from the liver of mice given either griseofulvin or isogriseofulvin. This porphyrin resembles closely in chromatographic and spectral properties the inhibitory pigment isolated after treatment with 3,5-diethoxycarbonyl-1,4-dihydrocollidine, but differs from other green pigments that do not inhibit protohaem ferro-lyase. A hypothesis is proposed to account for the differences in properties between the two groups of pigments and for the mechanism of inhibition of the enzyme.

Animals↗
PubMed1980

Substrate-dependent irreversible inactivation of cytochrome P-450: conversion of its haem moiety into modified porphyrins.

2-Allyl-2-isopropylacetamide and other drugs containing either an allyl, a vinyl or an ethynyl unsaturated side chain are metabolized by liver cytochrome P-450 to reactive derivatives that irreversibly inhibit the haemoprotein by a suicidal type of inactivation. The main target is the haem moiety of cytochrome P-450 which is converted into abnormal porphyrins. These have been isolated from the liver of treated rats, extensively purified and compared with model porphyrins. The abnormal porphyrins incorporate metal ions in vitro much more readily than does their parent porphyrin, protoporphyrin. They are also much more basic than protoporphyrin, and on titration with a strong acid they readily give rise to a porphyrin monocation which then requires relatively large amounts of acid for conversion to the porphyrin dication. In all these respects and also in the intensity of their bathochromic shifts these abnormal porphyrins closely resemble N-alkylated porphyrins and they markedly differ from porphyrins that are substituted at one of their meso-carbon positions or which bear electron-withdrawing substituents at the beta-positions of the pyrrole rings. This suggests strongly that reactive derivatives of the unsaturated drugs act as electrophilic reagents and alkylate one of the pyrrole nitrogen atoms of cytochrome P-450. A model centred on the apoprotein of cytochrome P-450 is considered for the degradation of liver haem caused by unsaturated drugs. The apocytochrome may accept exchangeable pools of liver haem for degradation, leading to a state of haem depletion and to activation of delta-aminolaevulinate synthase.

Acetamides↗
PubMed1979

Studies on the mechanism of experimental porphyria produced by 3,5-diethoxycarbonyl-1,4-dihydrocollidine. Role of a porphyrin-like inhibitor of protohaem ferro-lyase.

A green porphyrin-like pigment with inhibitory properties towards protohaem ferro-lyase activity was isolated from the livers of mice and rats after treatment with 3,5-diethoxycarbonyl-1,4-dihydrocollidine. Mice, which are more sensitive to the porphyrogenic properties of the drug, produce more inhibitor. The non-porphyrogenic analogue 3,5-diethoxycarbonylcollidine does not cause accumulation of the pigment in the liver. The inhibitory substance is present in control liver at low but measurable concentrations.

Animals↗
PubMed1979

Formation of cobalt protoporphyrin in the liver of rats. A mechanism for the inhibition of liver haem biosynthesis by inorganic cobalt.

1. Treatment of rats with small doses of CoCl2 decreases liver 5-aminolaevulinate synthase (EC 2.3.1.37) activity and impairs incorporation of 5-amino[14C]laevulinate into liver haem. Salts of other metals (cadmium, nickel, manganese and zinc) are all relatively inactive. 2. The dose-response curves obtained for both these effects closely mirror the accumulation in the liver of a compound that is labelled by 5-amino[14C]laevulinate and is unextractable by acetone/HCl. 3. Incorporation of 5-amino[14C]laevulinate into unextractable compound is also obtained in vitro by incubating liver homogenates with label in the presence of cobalt:isotope-dilution experiments show that the radioactivity passes through pools of porphobilinogen and protoporphyrin, but not of haem. 4. The unextractable compound is not covalently bound to protein and possesses the same extraction and spectral properties as authentic cobalt protoporphyrin. 5. It is concluded (a) that cobalt protoporphyrin is readily formed not only in vitro, but also in vivo, and (b) that its formation accounts for the impaired incorporation of 5-aminolaevulinate into haem and may also be responsible for the action of cobalt on 5-aminolaevulinate synthase.

5-Aminolevulinate Synthetase↗
PubMed1977

Loss of haem from cytochrome P-450 caused by lipid peroxidation and 2-allyl-2-isoprophylacetamide. An abnormal pathway not involving production of carbon monoxide.

1. Microsomal preparations undergoing lipid peroxidation produce CO and lose haem from cytochrome P-450. 2. The amount of CO produced does not correlate with the amount of haem lost and, after pre-labelling of microsomal haem in its bridges with 5-amino[5-14C]laevulinate, the radioactivity lost from haem is not recorved as CO. 3. Similarly, when pre-labelled microsomal haem is destroyed by the action of 2-allyl-2-isopropylacetamide, no radioactivity is recovered as CO. In clear contrast, on degradation of haem by the haem oxygenase system, CO is produced in an amount equimolar to the haem lost. 4. It is concluded that (a) the CO produced during lipid peroxidation originates from a source different from haem and (b) the degradations of haem caused by lipid peroxidation and 2-allyl-2-isopropylacetamide do not involve to any significant extent evolution of the methene-bridge carbon of haem as CO.

Acetamides↗
PubMed1977

Inhibition of haem synthesis caused by cobalt in rat liver. Evidence for two different sites of action.

Cobalt inhibits liver haem synthesis in vivo by acting at least two different sites in the biosynthetic pathway: (1) synthesis of 5-aminolaevulinate and (2) conversion of 5-amino-laevulinate into haem. The first effect is largely, if not entirely, due to inhibition of the activity of 5-aminolaevulinate synthase, rather than to inhibition of the formation of the enzyme. The second effect results from diversion of 5-aminolaevulinate into an unidentified liver pool with solubility properties similar to those of cobalt protoporphyrin.

5-Aminolevulinate Synthetase↗
PubMed1976

The effect of cobaltous chloride on liver haem metabolism in the rat. Evidence for inhibition of haem synthesis and for increased haem degradation.

After a single dose of cobaltous chloride there was a marked inhibition of liver 5-aminolaevulinate (5-ALA) synthetase (at 1 h) and this was followed in turn by a stimulation of haem oxygenase (at 3 h) and by a return of the synthetase activity to normal or above normal (at 17 h). Bile cannulation experiments were performed 1 and 17 h after administration of CoCl2. At 1 h there was a marked decrease in bile porphyrin content, no change in bile concentration of bilirubin, but a decrease in the conversion of [14C]-5-ALA to bilirubin and to liver haem. At 17 h, an the other hand, the bile excretion of both porphyrins and bilirubin was significantly greater than in controls and more radioactivity (from [14C]-5-ALA) appeared in the bile as bilirubin. It is concluded that the effects of cobalt on liver haem metabolism are complex and time-dependent. There is first inhibition of liver haem synthesis at two different steps of the pathway (synthesis of 5-ALA and conversion of 5-ALA to haem), with diversion of [14C]-5-ALA into a relatively stable liver pool different from haem; and at a later stage there is also an increase in the rate of liver haem degradation.

5-Aminolevulinate Synthetase↗
PubMed1975

Stimulation of the pathway of porphyrin synthesis in the liver of rats and mice by griseofulvin, 3,5-Diethoxycarbonyl-1,4-dihydrocollidine and related drugs: evidence for two basically different mechanisms.

Griseofulvin and isogriseofulvin cause, like 3,5-diethoxycarbonyl-1,4-dihydrocollidine, a fall in the activity of the hepatic enzyme porphyrin-metal chelatase and accumulation of protoporphyrin in the liver. Analogues of either griseofulvin or 3,5-diethoxycarbonyl-1,4-dihydrocollidine which do not decrease the chelatase activity are not porphyrogenic on their own, but can potentiate the porphyria caused by 3,5-diethoxycarbonyl-1,4-dihydrocollidine. This suggests the existence of two basically different mechanisms by which drugs stimulate the pathway of porphyrin synthesis in the liver.

Animals↗