The hepatic cytosolic hemoprotein tryptophan 2,3-dioxygenase (TDO) is the r
ate-limiting enzyme in tryptophan catabolism and thus plays a key role in r
egulating the physiological flux of tryptophan into relevant metabolic path
ways. The TDO protein is induced by corticosteroids such as dexamethasone (
DEX) and is stabilized by its prosthetic heme, In rats, acute chemically in
duced hepatic heme depletion reduces the functional hepatic TDO levels to 2
5-30% of basal levels within 1 h, and this decrease persists beyond 28 h of
heme depletion at which time only 25-30% of the protein is available for h
eme incorporation. Since this could stem from impaired de novo synthesis an
d/or instability of the newly synthesized apoTDO protein in the absence of
heme, we examined the specific role of heme in these events in a previously
validated rat model of acute hepatic heme depletion triggered by the P450
suicide substrate 3,5-dicarbethoxy 2,6-dimethyl-4-ethyl-1,4-dihydropyridine
. We now show that exogenous heme can reverse the functional impairment of
the enzyme observed during hepatic heme depletion and fully restore the imp
aired DEX-mediated induction of the enzyme to normal. Furthermore, through
Northern/slot blot analyses coupled with nuclear run-on studies, we now doc
ument that this heme regulation of TDO is exerted primarily at the transcri
ptional level. Immunoblotting analyses also reveal corresponding changes in
the TDO protein, thereby establishing that heme is necessary for DEX-induc
ible TDO mRNA transcription and subsequent translation. Thus, the TDO gene
may contain heme-regulatory elements in addition to the reported glucocorti
coid-responsive elements. Together, these findings suggest that clinically,
hepatic heme deficiency may enhance the tryptophan flux into synthetic (se
rotonergic) pathways, not only by depriving prosthetic heme for a functiona
lly competent TDO hemoprotein, its primary catabolic enzyme, but also by im
pairing the de novo synthesis of this enzyme. (C) 2000 Academic Press.