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G Srivastava

Publications and source records attributed to G Srivastava.

At least 163 records · Page 9Linked to original sources
PubMed1982

Radiochemical assay for gamma-aminolevulinate synthase.

We describe here a standard protocol for the radiochemical assay of gamma-aminolevulinate synthase from 17-day chick embryo liver mitochondria and starved male rat liver mitochondria. The assay is sensitive to at least 15 pmol of the gamma-aminolevulinate produced and can be adapted to suit most tissue types. A number of possible modifications to the procedure are discussed.

5-Aminolevulinate Synthetase↗
PubMed1980

Evidence that in chick embryos destruction of hepatic microsomal cytochrome P-450 haem is a general mechanism of induction of delta-aminolaevulinate synthase by porphyria-causing drugs.

A variety of prophyrinogenic compounds were tested for their effect in ovo on chick-embryo liver microsomal cytochrome P-450 haem concentration and mitochondrial delta-aminolaevulinate synthase activity. With all drugs tested, there was a 30--50% decrease in cytochrome P-450 haem concentration within 1 h of treatment, and this was closely followed by an increase in delta-aminolaevulinate synthase activity. The relationship was independent of the extent of enzyme induction and is consistent with the proposal that drug-mediated destruction of cytochrome P-450 haem is the primary mechanism of induction of delta-aminolaevulinate synthase. After induction, synthesis of delta-aminolaevulinate synthase could be maintained by inhibiting further haem synthesis. These studies suggest that induction of porphyria is a combination of two distinct processes: (a) induction of delta-aminolaevulinate synthase synthesis by destruction of cytochrome P-450 haem and consequent depletion of cellular free haem; (b) maintenance of continued delta-aminolaevulinate synthase synthesis by preventing replenishment of cellular haem either by inhibiting haem synthesis and/or by promoting continuous removal of newly synthesized haem.

5-Aminolevulinate Synthetase↗
PubMed1980

Haem control in experimental porphyria. The effect of haemin on the induction of delta-aminolaevulinate synthase in isolated chick-embryo liver cells.

2-Allyl-2-isopropylacetamide-mediated induction of hepatic porphyria was studied in isolated chick-embryo liver cells. Increased delta-aminolaevulinate synthase activity occurred within 1h of induction and continued to increase for 8h. Protoporphyrins synthesized during this time accumulated to a concentration 10-fold greater than that in the control. Removal of 2-allyl-2-isopropylacetamide from the cells by washing at 3h immediately inhibited further increases in delta-aminolaevulinate synthase synthesis. However substitution of 2-allyl-2-isopropylacetamide at 3h by deferoxamine methane-sulphonate, an inhibitor of haem synthesis, allowed continued delta-aminolaevulinate synthase induction at an unaltered rate, even though this agent did not, by itself, induce enzyme synthesis. Exogenously added haemin was shown completely to inhibit 2-allyl-2-isopropylacetamide-mediated delta-aminolaevulinate synthase induction at concentrations as low as 20nm, a value that is less than the reported physiological one. The duration of inhibition was dependent on the concentration of added haemin and was followed by a period of delta-aminolaevulinate synthase synthesis at a rate similar to that of the control. These data are consistent with the hypothesis that delta-aminolaevulinate synthase synthesis is regulated by the concentration of intracellular haem and that induction is initiated by 2-allyl-2-isopropylacetamide-mediated destruction of haem. Induction of delta-aminolaevulinate synthase was shown to be dependent on both RNA and protein synthesis, and a study of the comparative effects of cordycepin, cycloheximide and haem has shown that, at haemin concentrations up to 50nm, the inhibition of delta-aminolaevulinate synthase synthesis followed kinetics similar to the effect of cordycepin, with no synergism between cordycepin and 50nm-haemin. However, at a haemin concentration of 2mum, the inhibition of delta-aminolaevulinate synthase synthesis followed similar kinetics to the effect of cycloheximide. These data demonstrate the control of delta-aminolaevulinate synthase synthesis by low concentrations of haemin and suggests that the primary effect of haemin is at the level of transcription.

5-Aminolevulinate Synthetase↗