Cobaltous chloride-mediated induction of rat hepatic tryptophan 2,3-dioxygenase: implications for the use of the enzyme to probe the hepatic free heme pool.
Subcutaneous administration of CoCl2, a well recognized inhibitor of hepatic heme synthesis, to rats results in the functional stimulation of total (holo- + apo) tryptophan 2,3 dioxygenase (TDO), a hemoprotein and the key rate-limiting enzyme in the oxidative metabolism of tryptophan to formylkynurenine. Because basal holo-TDO activity is not altered, TDO stimulation appears to be entirely due to CoCl2-mediated increase of its apoprotein. This apoTDO increase was blocked by conventional inhibitors of protein synthesis (actinomycin D, cycloheximide), thereby revealing that such CoCL2-mediated apoprotein increase truly reflected TDO induction. To determine whether the CoCl2-mediated TDO induction involved the action of its natural physiological inducers (glucocorticoids) or was due to direct CoCl2-regulation of the TDO gene, rats were adrenalectomized before CoCl2 administration. In adrenalectomized rats, CoCl2 failed to induce TDO, but induction was completely restored on administration of the glucocorticoid hydrocortisone, but not of adrenaline. These findings reveal that CoCl2-mediated TDO induction is indirect and entails glucocorticoid participation. In addition, because CoCl2 lowered the % heme saturation of TDO [= 100(holo TDO activity/total (apo+holo) TDO activity] largely by increasing the apoTDO protein levels rather than by affecting the basal holo-TDO levels (as expected from its inhibition of heme synthesis), these findings question the widely accepted use of the relative intrahepatic % heme saturation of TDO as a reporter of the hepatic "free" heme pool.
Department of Pharmacology, University of California, San Francisco 94143.
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D000311:Adrenal Glands / Q000502:physiology; D000315:Adrenalectomy; D000818:Animals; D001051:Apoenzymes / Q000096:biosynthesis; D003035:Cobalt / Q000494:pharmacology*; D004790:Enzyme Induction / Q000187:drug effects; D006418:Heme / Q000378:metabolism*; D006419:Heme Oxygenase (Decyclizing) / Q000378:metabolism; D008099:Liver / Q000187:drug effects* / Q000378:metabolism*; D008297:Male; D008670:Metals / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D014366:Tryptophan Oxygenase / Q000096:biosynthesis*
D001051:Apoenzymes; D008670:Metals; D003035:Cobalt; D006418:Heme; D014366:Tryptophan Oxygenase; D006419:Heme Oxygenase (Decyclizing); C018021:cobaltous chloride