Effect of endotoxin on tryptophan pyrrolase and delta-aminolaevulinate synthase: evidence for an endogenous regulatory haem fraction in rat liver

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Abstract
Endotoxin was administered to rats at a dose shown previously to stimulate hepatic heme oxygenase activity and to block induction of .delta.-aminolevulinate synthase EC 2.3.1.37, apparently by causing redistribution of heme from cytochrome P-450 to a regulatory heme pool in the liver. Within 5 h of the administration of endotoxin (at a time when the effect of the compound on cytochrome P-450 is maximal) the relative saturation of tryptophan pyrrolase EC 1.13.11.11 with intrinsic heme rose from a basal value of 50% to 90%, indicating that free heme had become available. Concurrently, the activity of .delta.-aminolevulinate synthase was decreased to 25% of its basal value. Heme oxygenase reached peak activity 13 h after endotoxin administration. New evidence was provided for the existence of an unassigned hepatic heme fraction which exchanges with cytochrome P-450 heme and regulates these 3 enzyme functions.