Tryptophan pyrrolase in haem regulation. The mechanisms of enhancement of rat liver 5-aminolaevulinate synthase activity by starvation and of the glucose effect on induction of the enzyme by 2-allyl-2-isopropylacetamide.
1. Rat liver tryptophan pyrrolase activity is enhanced by a hormonal-type mechanism during the first 2 days of starvation and by a substrate-type mechanism during the subsequent 2 days. 5-Aminolaevulinate synthase activity is also enhanced during the first 2 days of starvation, but returns thereafter to values resembling those observed in the fed rat. Treatments that prevent or reversé the enhancement of tryptophan pyrrolase activity in 24-48h-starved rats also abolish that of 5-aminolaevulinate synthase activity. Starvation of guinea pigs, which does not enhance the pyrrolase activity, also fails to alter that of the synthase. It is suggested that the decrease in 5-aminolaevulinate synthase activity in 72-96h-starved rats represents negative-feedback repression of synthesis, possibly involving tryptophan participation, whereas the enhancement observed in 24-48h-starved animals is caused by positive-feedback induction secondarily to increased utilization of the regulatory-haem pool by the newly synthesized apo-(tryptophan pyrrolase). 2. Glucose, fructose and sucrose abolish the 24h-starvation-induced increases in rat liver tryptophan pyrrolase and 5-aminolaevulinate synthase activities. Cortisol reverses the glucose effect on 5-aminolaevulinate synthase activity, presumably by enabling pyrrolase to re-utilize the regulatory-haem pool after induction of synthesis of this latter enzyme. 3. The impaired ability of 2-allyl-2-isopropylacetamide to enhance markedly 5-aminolaevulinate synthase activity in 24h-starved rats treated with glucose is associated with a failure of the porphyrogen to cause loss of tryptophan pyrrolase haem. Cortisol restores the ability of the porphyrogen to destroy tryptophan pyrrolase haem and to enhance markedly 5-aminolaevulinate synthase activity, presumably by enhancing tryptophan pyrrolase synthesis and, thereby, its re-utilization of the regulatory-haem pool. It is tentatively suggested that 2-allyl-2-isopropylacetamide destroys the above pool only after it has become bound to (or utilized by) apo-(tryptophan pyrrolase).
D000624:5-Aminolevulinate Synthetase / Q000378:metabolism*; D000081:Acetamides / Q000494:pharmacology*; D000502:Allylisopropylacetamide / Q000494:pharmacology*; D000818:Animals; D004790:Enzyme Induction / Q000187:drug effects; D005632:Fructose / Q000494:pharmacology; D005947:Glucose / Q000494:pharmacology; D006418:Heme / Q000378:metabolism*; D006854:Hydrocortisone / Q000494:pharmacology; D008099:Liver / Q000187:drug effects / Q000201:enzymology*; D008297:Male; D051381:Rats; D011919:Rats, Inbred Strains; D013217:Starvation / Q000201:enzymology; D013395:Sucrose / Q000494:pharmacology; D014366:Tryptophan Oxygenase / Q000378:metabolism*
D000081:Acetamides; D000502:Allylisopropylacetamide; D005632:Fructose; D006418:Heme; D013395:Sucrose; D014366:Tryptophan Oxygenase; D000624:5-Aminolevulinate Synthetase; D005947:Glucose; D006854:Hydrocortisone